U.S. patent number 8,450,144 [Application Number 12/722,795] was granted by the patent office on 2013-05-28 for semiconductor device and method for manufacturing the same.
This patent grant is currently assigned to Semiconductor Energy Laboratory Co., Ltd.. The grantee listed for this patent is Hideyuki Kishida, Hiroki Ohara, Junichiro Sakata, Toshinari Sasaki, Shunpei Yamazaki. Invention is credited to Hideyuki Kishida, Hiroki Ohara, Junichiro Sakata, Toshinari Sasaki, Shunpei Yamazaki.
United States Patent |
8,450,144 |
Sakata , et al. |
May 28, 2013 |
Semiconductor device and method for manufacturing the same
Abstract
An object of an embodiment of the present invention is to
provide a semiconductor device provided with a thin film transistor
which includes an oxide semiconductor layer and has high electric
characteristics. The semiconductor device includes a gate electrode
over an insulating surface, an oxide semiconductor layer including
silicon oxide, an insulating layer between the gate electrode and
the oxide semiconductor layer, and source and drain regions between
the oxide semiconductor layer including silicon oxide and source
and drain electrode layers. The source and drain regions are formed
using a degenerate oxide semiconductor material or a degenerate
oxynitride material.
Inventors: |
Sakata; Junichiro (Atsugi,
JP), Kishida; Hideyuki (Atsugi, JP), Ohara;
Hiroki (Sagamihara, JP), Sasaki; Toshinari
(Atsugi, JP), Yamazaki; Shunpei (Setagaya,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Sakata; Junichiro
Kishida; Hideyuki
Ohara; Hiroki
Sasaki; Toshinari
Yamazaki; Shunpei |
Atsugi
Atsugi
Sagamihara
Atsugi
Setagaya |
N/A
N/A
N/A
N/A
N/A |
JP
JP
JP
JP
JP |
|
|
Assignee: |
Semiconductor Energy Laboratory
Co., Ltd. (Atsugi-shi, Kanagawa-ken, JP)
|
Family
ID: |
42772193 |
Appl.
No.: |
12/722,795 |
Filed: |
March 12, 2010 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
|
US 20100244020 A1 |
Sep 30, 2010 |
|
Foreign Application Priority Data
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|
|
|
Mar 26, 2009 [JP] |
|
|
2009-077386 |
|
Current U.S.
Class: |
438/104;
257/E21.46 |
Current CPC
Class: |
H01L
21/02592 (20130101); H01L 27/1288 (20130101); H01L
29/66969 (20130101); H01L 21/02554 (20130101); H01L
21/02631 (20130101); H01L 29/7869 (20130101); H01L
21/02565 (20130101); H01L 27/1225 (20130101); H01L
29/78618 (20130101); H01L 27/12 (20130101); G09G
2300/0861 (20130101); H01L 27/3262 (20130101); G09G
2310/0297 (20130101); G09G 3/3275 (20130101); H01L
27/3276 (20130101); G09G 2310/0248 (20130101); G09G
3/3258 (20130101); G09G 2300/0842 (20130101) |
Current International
Class: |
H01L
21/00 (20060101) |
Field of
Search: |
;438/104,151,778
;257/E21.46 |
References Cited
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WO |
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WO-2007/119386 |
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Oct 2007 |
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WO |
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|
Primary Examiner: Matthews; Colleen
Attorney, Agent or Firm: Robinson; Eric J. Robinson
Intellectual Property Law Office, P.C.
Claims
What is claimed is:
1. A method for manufacturing a semiconductor device comprising:
forming a gate electrode over an insulating surface; forming an
insulating layer over the gate electrode; forming an oxide
semiconductor layer including silicon oxide over the insulating
layer by a sputtering method using a first oxide semiconductor
target including silicon oxide at 2.5 wt % to 20 wt % inclusive;
and forming an oxynitride layer over the oxide semiconductor layer
including silicon oxide in an atmosphere including nitrogen using a
second oxide semiconductor target to form a source region and a
drain region.
2. The method for manufacturing a semiconductor device according to
claim 1, further comprising a step of removing part of the
oxynitride layer, which overlaps with the gate electrode so that
part of the oxide semiconductor layer including silicon oxide is
exposed, after forming the oxynitride layer.
3. The method for manufacturing a semiconductor device according to
claim 1, wherein the semiconductor device is one selected from the
group consisting of an electronic book, a television set, an
amusement machine, and a phone.
4. A method for manufacturing a semiconductor device comprising:
forming an oxide semiconductor layer over an insulating surface by
a sputtering method using a first oxide semiconductor target
including silicon oxide at 2.5 wt % to 20 wt % inclusive; forming
an oxynitride layer over the oxide semiconductor layer including
silicon oxide by a sputtering method using a second oxide
semiconductor target in an atmosphere including nitrogen to form a
source region and a drain region; forming an insulating layer
covering the oxynitride layer; and forming a gate electrode over
the insulating layer.
5. The method for manufacturing a semiconductor device according to
claim 4, wherein the semiconductor device is one selected from the
group consisting of an electronic book, a television set, an
amusement machine, and a phone.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having a
circuit formed using a thin film transistor (hereinafter, referred
to as TFT) and a manufacturing method thereof. For example, the
present invention relates to an electro-optical device typified by
a liquid crystal display panel, or an electronic device which has a
light-emitting display device including an organic light-emitting
element as a component.
In this specification, a semiconductor device generally means a
device which can function by utilizing semiconductor
characteristics, and an electro-optic device, a semiconductor
circuit, and electronic equipment are all semiconductor
devices.
2. Description of the Related Art
Various metal oxides are used for a variety of applications. Indium
oxide is a well-known material and is used as a transparent
electrode material which is necessary for liquid crystal displays
and the like.
Some metal oxides have semiconductor characteristics. Metal oxides
having semiconductor characteristics are a kind of compound
semiconductor. The compound semiconductor is a semiconductor formed
using two or more kinds of atoms bonded together. In general, metal
oxides become insulators. However, it is known that metal oxides
become semiconductors depending on the combination of elements
included in the metal oxides.
For example, it is known that tungsten oxide, tin oxide, indium
oxide, zinc oxide, and the like are metal oxides which have
semiconductor characteristics. A thin film transistor in which a
transparent semiconductor layer which is formed using such a metal
oxide serves as a channel formation region is disclosed (Patent
Documents 1 to 4 and Non-Patent Document 1).
Further, not only single-component oxides but also multi-component
oxides are known as metal oxides. For example,
InGaO.sub.3(ZnO).sub.m (m is a natural number) which is a
homologous compound is a known material (Non-Patent Documents 2 to
4).
Furthermore, it is confirmed that such an In--Ga--Zn-based oxide is
applicable to a channel layer of a thin film transistor (Patent
Document 5 and Non-Patent Documents 5 and 6).
Further, attention has been drawn to a technique for manufacturing
a thin film transistor using an oxide semiconductor, and applying
the thin film transistor to an electronic device or an optical
device. For example, Patent Document 6 and Patent Document 7
disclose a technique by which a thin film transistor is
manufactured using zinc oxide or an In--Ga--Zn--O-based oxide
semiconductor as an oxide semiconductor film and such a thin film
transistor is used as a switching element or the like of an image
display device.
REFERENCE
Patent Documents
[Patent Document 1] Japanese Published Patent Application No.
S60-198861
[Patent Document 2] Japanese Published Patent Application No.
H8-264794
[Patent Document 3] Japanese Translation of PCT International
Application No. H11-505377
[Patent Document 4] Japanese Published Patent Application No.
2000-150900
[Patent Document 5] Japanese Published Patent Application No.
2004-103957
[Patent Document 6] Japanese Published Patent Application No.
2007-123861
[Patent Document 7] Japanese Published Patent Application No.
2007-096055
Non-Patent Document
[Non-Patent Document 1] M. W. Prins, K. O. Grosse-Holz, G. Muller,
J. F. M. Cillessen, J. B. Giesbers, R. P. Weening, and R. M. Wolf,
"A ferroelectric transparent thin-film transistor", Appl. Phys.
Lett., 17 Jun. 1996, Vol. 68, p. 3650-3652
[Non-Patent Document 2] M. Nakamura, N. Kimizuka, and T. Mohri,
"The Phase Relations in the In.sub.2O.sub.3--Ga.sub.2ZnO.sub.4--ZnO
System at 1350.degree. C.", J. Solid State Chem., 1991, Vol. 93, p.
298-315
[Non-Patent Document 3] N. Kimizuka, M. Isobe, and M. Nakamura,
"Syntheses and Single-Crystal Data of Homologous Compounds,
In.sub.2O.sub.3(ZnO).sub.m (m=3, 4, and 5), InGaO.sub.3(ZnO).sub.3,
and Ga.sub.2O.sub.3(ZnO).sub.m (m=7, 8, 9, and 16) in the
In.sub.2O.sub.3--ZnGa.sub.2O.sub.4--ZnO System", J. Solid State
Chem., 1995, Vol. 116, p. 170-178
[Non-Patent Document 4] M. Nakamura, N. Kimizuka, T. Mohri, and M.
Isobe, "Syntheses and crystal structures of new homologous
compounds, indium iron zinc oxides (InFeO.sub.3(ZnO).sub.m)
(m=natural number) and related compounds", KOTAI BUTSURI (SOLID
STATE PHYSICS), 1993, Vol. 28, No. 5, p. 317-327
[Non-Patent Document 5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M.
Hirano, and H. Hosono, "Thin-film transistor fabricated in
single-crystalline transparent oxide semiconductor", SCIENCE, 2003,
Vol. 300, p. 1269-1272
[Non-Patent Document 6] K. Nomura, H. Ohta, A. Takagi, T. Kamiya,
M. Hirano, and H. Hosono, "Room-temperature fabrication of
transparent flexible thin-film transistors using amorphous oxide
semiconductors", NATURE, 2004, Vol. 432, p. 488-492
SUMMARY OF THE INVENTION
An object of an embodiment of the present invention is to provide a
semiconductor device including a thin film transistor which
includes an oxide semiconductor layer and has high electric
characteristics.
In order to realize an amorphous oxide semiconductor layer, a thin
film transistor which includes an oxide semiconductor layer
including silicon oxide or silicon oxynitride is provided.
Typically, an oxide semiconductor layer is formed using an oxide
semiconductor target including silicon oxide at 2.5 wt % to 20 wt %
inclusive, preferably, at 7.5 wt % to 12.5 wt % inclusive, and
silicon oxide (SiO.sub.x) which hinders crystallization is added to
the oxide semiconductor layer, whereby the thin film transistor is
obtained, whose gate threshold voltage at which a channel is formed
is positive and as close to 0 V as possible.
An embodiment of the present invention disclosed in this
specification is a semiconductor device including a gate electrode
over an insulating surface, an oxide semiconductor layer including
silicon oxide, an insulating layer between the gate electrode and
the oxide semiconductor layer, and source and drain regions between
the oxide semiconductor layer including silicon oxide and source
and drain electrode layers. In the semiconductor device, the source
and drain regions are formed using a degenerate oxide semiconductor
material or a degenerate oxynitride material.
The oxide semiconductor layer including silicon oxide is formed
using a Zn--O-based oxide semiconductor, an In--Ga--Zn--O-based
oxide semiconductor, an In--Sn--Zn--O-based oxide semiconductor, a
Ga--Sn--Zn--O-based oxide semiconductor, an In--Zn--O-based oxide
semiconductor, a Sn--Zn--O-based oxide semiconductor, an
In--Sn--O-based oxide semiconductor, or a Ga--Zn--O-based oxide
semiconductor.
In order to reduce contact resistance with source and drain
electrode layers which are formed from a metal material with low
electric resistance, the source and drain regions are formed
between the source and drain electrode layers and the oxide
semiconductor layer including silicon oxide.
In addition, in order to form an ohmic contact, the source or drain
region (a buffer layer) whose carrier concentration is higher than
that of the oxide semiconductor layer is intentionally formed
between the oxide semiconductor layer and the source electrode
layer (or the drain electrode layer). Note that the source and
drain regions have n-type conductivity and can be referred to as
n.sup.+ regions. In the case where the source and drain regions are
referred to as the n.sup.+ regions (N.sup.+-type regions), the
oxide semiconductor layer which is made to function as a channel
formation region can also be referred to as an i-type region (an
I-type region) in contrast to the n.sup.+ regions. An NI junction
is formed by provision of the source and drain regions, so that a
semiconductor device provided with a thin film transistor having a
short channel length of 5 .mu.m or less and high field effect
mobility can be obtained.
Further, the source and drain regions (also referred to as the
N.sup.+-type regions, n.sup.+ layers, or buffer layers) are
preferably formed using a degenerate oxide semiconductor. The
degenerate oxide semiconductor preferably transmits light. The
oxide semiconductor layer is formed using a Zn--O-based oxide
semiconductor, an In--Ga--Zn--O-based oxide semiconductor, an
In--Zn--O-based oxide semiconductor, a Sn--Zn--O-based oxide
semiconductor, an In--Sn--O-based oxide semiconductor, an
Al--Zn--O-based oxide semiconductor or a Ga--Zn--O-based oxide
semiconductor. Alternatively, the source and drain regions may be
formed using a Zn--O-based non-single-crystal film including
nitrogen, i.e., a Zn--O--N-based non-single-crystal film (also
referred to as a ZnON film); or an In--Ga--Zn--O-based
non-single-crystal film including nitrogen, i.e., an
In--Ga--Zn--O--N-based non-single-crystal film (also referred to as
an IGZON film). Further alternatively, the source and drain regions
may be formed using a Ga--Zn--O-based non-single-crystal film; or a
Ga--Zn--O-based non-single-crystal film including nitrogen, i.e., a
Ga--Zn--O--N-based non-single-crystal film. Further alternatively,
the source and drain regions may be formed using an Al--Zn--O-based
non-single-crystal film; or an Al--Zn--O-based non-single-crystal
film including nitrogen, i.e., an Al--Zn--O--N-based
non-single-crystal film. Note that each of the Ga--Zn--O-based
oxide semiconductor and the Ga--Zn--O--N-based oxide semiconductor
preferably includes gallium at 1 wt % to 10 wt % inclusive, and
each of the Al--Zn--O-based oxide semiconductor and the
Al--Zn--O--N-based oxide semiconductor preferably includes aluminum
at 1 wt % to 10 wt % inclusive. Further alternatively, a
Zn--O--N-based non-single-crystal film, which includes nitrogen, or
a Sn--Zn--O--N-based non-single-crystal film, which includes
nitrogen, may be used.
As a material of the source and drain electrode layers, there are
an element selected from Al, Cr, Ta, Ti, Mo, and W, an alloy
including any of the elements as a component, an alloy film
including a combination of any of the elements, and the like.
Alternatively, indium tin oxide, indium tin oxide including silicon
oxide, aluminum doped zinc oxide (AZO) or gallium doped zinc oxide
(GZO) can be used.
Note that the oxide semiconductor layer including silicon oxide is
formed by a sputtering method using an oxide semiconductor target
including silicon oxide at 2.5 wt % to 20 wt % inclusive.
In particular, in the case where the source or drain region (the
buffer layer) whose carrier concentration is higher than that of
the oxide semiconductor layer is intentionally provided between the
oxide semiconductor layer and the source electrode layer (or the
drain electrode layer), there is a possibility that the buffer
layer is also damaged by an electric charge caused by formation of
plasma and resistance of the buffer layer is increased, so that the
buffer layer cannot exhibit its own function.
Further, there is a possibility that characteristics of the oxide
semiconductor layer are changed or reliability thereof is reduced
due to reaction of the oxide semiconductor layer with moisture,
hydrogen ions, OH.sup.- (also referred to as an OH group) or the
like.
Thus, a resin layer having good flatness is formed as a first
protective insulating film covering the oxide semiconductor layer,
and then a second protective insulating film is formed over the
resin layer by a sputtering method or a plasma CVD method under a
low power condition. In this manner, different protective
insulating films are stacked, whereby a semiconductor device having
long-term reliability, in which plasma damage to the oxide
semiconductor layer is reduced and sealing property is greatly high
can be obtained.
Further, a second gate electrode covers the oxide semiconductor
layer, so that the second gate electrode has a blocking function
against moisture, hydrogen ions, OH.sup.-, or the like. In the case
where a conductive film which blocks light is used as the second
gate electrode, the second gate electrode has an effect of
preventing electric characteristics of the thin film transistor
from changing due to photosensitivity of the oxide semiconductor
and has an effect of stabilizing the electric characteristics of
the thin film transistor.
An embodiment of the present invention, which realizes the above
structure is a method for manufacturing a semiconductor device,
including the steps of: forming a gate electrode over an insulating
surface; forming an insulating layer over the gate electrode;
forming an oxide semiconductor layer including silicon oxide over
that insulating layer by a sputtering method using a first oxide
semiconductor target including silicon oxide at 2.5 wt % to 20 wt %
inclusive; and forming an oxynitride layer over the oxide
semiconductor layer including silicon oxide in an atmosphere
including nitrogen using a second oxide semiconductor target.
In the manufacturing method, a channel-etch type thin film
transistor is manufactured in such a manner that part of the
oxynitride layer, which overlaps with the gate electrode layer is
removed after formation of the oxynitride layer so that part of the
oxide semiconductor layer including silicon oxide is exposed.
The thin film transistor of the present invention is not limited to
the channel-etch type thin film transistor, but a bottom-gate type
thin film transistor, a bottom-contact type thin film transistor,
or a top-gate type thin film transistor can be formed.
Another embodiment of the present invention is a method for
manufacturing a top-gate thin film transistor, including the steps
of: forming an oxide semiconductor layer over an insulating surface
by a sputtering method using a first oxide semiconductor target
including silicon oxide at 2.5 wt % to 20 wt % inclusive; forming
an oxynitride layer over the oxide semiconductor layer including
silicon oxide by a sputtering method using a second oxide
semiconductor target in an atmosphere including nitrogen; forming
an insulating layer covering the oxynitride layer; and forming a
gate electrode over the insulating layer.
In each of the above manufacturing methods, the oxynitride layer is
used as source and drain regions which are provided between source
and drain electrode layers and the oxide semiconductor layer
including silicon oxide in order to reduce contact resistance with
the source and drain electrode layers formed using a metal material
with low electric resistance value.
In addition, in the case where ions, in particular, hydrogen
radicals are included in plasma at the time of generation of the
plasma after formation of the oxide semiconductor layer, there is a
possibility that a surface of the oxide semiconductor layer, which
is exposed to plasma, is damaged. Further, there is a possibility
that the oxide semiconductor layer is also damaged by an electric
charge at the time of generation of plasma performed after
formation of the oxide semiconductor layer.
In particular, in the case where the buffer layer whose carrier
concentration is higher than that of the oxide semiconductor layer
(the source or drain region) is intentionally provided between the
oxide semiconductor layer and the source electrode layer (or the
drain electrode layer), the buffer layer is also damaged by an
electric charge caused by formation of plasma and resistance of the
buffer layer is increased, so that there is a possibility that the
buffer layer cannot exhibit its own function.
Further, there is a possibility that characteristics of the oxide
semiconductor layer are changed or reliability thereof is reduced
due to reaction of the oxide semiconductor layer and moisture,
hydrogen ions, OH.sup.- or the like.
Thus, a resin layer having good flatness is formed as a first
protective insulating film covering the oxide semiconductor layer,
and then a second protective insulating film formed by a sputtering
method or a plasma CVD method under a low power condition is formed
over the resin layer. In this manner, different protective
insulating films are stacked, whereby a semiconductor device having
long-term reliability, in which plasma damage to the oxide
semiconductor layer is reduced and sealing property is greatly high
can be obtained.
Further, a second gate electrode covers the oxide semiconductor
layer, so that the second gate electrode also has a blocking
function against moisture, hydrogen ions, OH.sup.-, or the like. In
the case where a conductive film which blocks light is used as the
second gate electrode, the second gate electrode has an effect of
preventing electric characteristics of the thin film transistor
from changing due to photosensitivity of the oxide semiconductor
and stabilizing the electric characteristics of the thin film
transistor.
Further, a base film is preferably formed over the insulating
surface of a glass substrate or the like. For example, a silicon
nitride film or a silicon nitride oxide film is provided. In this
case, these films can be function as an etching stopper which
prevents the glass substrate from being etched when a first gate
electrode is selectively etched so that a top surface thereof has a
desired shape. In addition, the base film has a blocking function
against moisture, hydrogen ions, OH.sup.-, or the like. In this
manner, when the films having a blocking function against moisture,
hydrogen ions, OH.sup.-, or the like are provided to lie over,
below, and around the oxide semiconductor layer, a semiconductor
device having a still higher sealing property and long-term
reliability can be obtained.
A term indicating a direction such as "on", "over", "under",
"below", "side", "horizontal", or "perpendicular" in this
specification is based on the assumption that a device is provided
over a substrate surface.
An object of the present invention is to realize a semiconductor
device including a thin film transistor which includes an oxide
semiconductor layer including silicon oxide and has excellent
electric characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1C are cross-sectional views and a top view each
illustrating an embodiment of the present invention;
FIGS. 2A and 2B are a cross-sectional view and a top view, which
illustrate an embodiment of the present invention;
FIG. 3 is a cross-sectional view illustrating an embodiment of the
present invention;
FIG. 4 is a top view illustrating an embodiment of the present
invention;
FIGS. 5A-1 and 5A-2 and FIGS. 5B-1 and 5B-2 are cross-sectional
views and top views, which illustrate an embodiment of the present
invention;
FIG. 6 is a top view illustrating an embodiment of the present
invention;
FIG. 7 illustrates a model of a single crystal structure of
ZnO;
FIGS. 8A to 8E are graphs each showing a radical distribution
function g(r) of a model;
FIGS. 9A to 9D are graphs each showing a radical distribution
function g(r) of a model;
FIGS. 10A to 10E are graphs each showing a result by simulation of
XRD of a model;
FIGS. 11A to 11D are graphs each showing a result by simulation of
XRD of a model;
FIGS. 12A to 12E are cross-sectional views illustrating
manufacturing steps of an embodiment of the present invention;
FIGS. 13A and 13B are a cross-sectional view and a top view which
illustrate an embodiment of the present invention;
FIGS. 14A and 14B are a cross-sectional view and a top view which
illustrate an embodiment of the present invention;
FIGS. 15A and 15B are a cross-sectional view and a top view which
illustrate an embodiment of the present invention;
FIGS. 16A and 16B are a cross-sectional view and a top view which
illustrate an embodiment of the present invention;
FIGS. 17A and 17B are each a block diagram showing a semiconductor
device of an embodiment of the present invention;
FIG. 18 is a diagram of a structure of a signal line driver circuit
illustrating an embodiment of the present invention;
FIG. 19 is a timing chart of an operation of a signal line driver
circuit illustrating an embodiment of the present invention;
FIG. 20 is a timing chart of an operation of a signal line driver
circuit illustrating an embodiment of the present invention;
FIG. 21 is a diagram showing one example of a structure of a shift
register illustrating an embodiment of the present invention;
FIG. 22 is a diagram showing a connection structure of a flip-flop
shown in FIG. 21;
FIG. 23 is a diagram of an equivalent circuit of a pixel of a
semiconductor device illustrating an embodiment of the present
invention;
FIGS. 24A to 24C are each a cross-sectional view of a semiconductor
device illustrating an embodiment of the present invention;
FIGS. 25A and 25B are a top view and a cross-sectional view of a
semiconductor device illustrating an embodiment of the present
invention;
FIGS. 26A-1 and 26A-2 are top views and FIG. 26B is a
cross-sectional view of a semiconductor device illustrating an
embodiment of the present invention;
FIG. 27 is a cross-sectional view of a semiconductor device
illustrating an embodiment of the present invention;
FIGS. 28A and 28B are a cross-sectional view and an external view
of an electronic appliance which illustrate a semiconductor device
of an embodiment of the present invention;
FIGS. 29A and 29B each illustrate an electronic appliance of an
embodiment of the present invention;
FIGS. 30A and 30B each illustrate an electronic appliance of an
embodiment of the present invention;
FIGS. 31A and 31B are cross-sectional views each illustrating an
embodiment of the present invention;
FIGS. 32A to 32D are cross-sectional views each illustrating an
embodiment of the present invention;
FIGS. 33A and 33B are cross-sectional views each illustrating an
embodiment of the present invention;
FIGS. 34A and 34B are cross-sectional views each illustrating an
embodiment of the present invention;
FIGS. 35A to 35D are cross-sectional views each illustrating an
embodiment of the present invention;
FIGS. 36A to 36D are cross-sectional views each illustrating an
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described
in detail with reference to the accompanying drawings. However, the
present invention is not limited to the description below, and it
is easily understood by those skilled in the art that modes and
details disclosed herein can be modified in various ways without
departing from the spirit and the scope of the present invention.
Therefore, the present invention is not construed as being limited
to description of the embodiments.
Embodiment 1
This embodiment describes an example of a thin film transistor
using an oxide semiconductor layer including silicon oxide with
reference to FIGS. 1A, 1B, and 1C.
FIG. 1A illustrates a thin film transistor 160 which is one type of
a bottom-gate structure, and is a cross-sectional view of a
structure called a channel-etch type. FIG. 1B illustrates an
example of a top view of the thin film transistor whose cross
section taken along line B1-B2 corresponds to FIG. 1A.
The thin film transistor 160 illustrated in FIG. 1A includes a gate
electrode layer 101 over a substrate 100, a gate insulating layer
102 over the gate electrode layer 101, and an oxide semiconductor
layer 103 including silicon oxide which is over the gate insulating
layer 102 and overlaps with the gate electrode layer 101. In
addition, source and drain electrode layers 105a and 105b which
overlap with part of the oxide semiconductor layer 103 including
silicon oxide are provided, and source and drain regions 104a and
104b are provided between part of the oxide semiconductor layer 103
including silicon oxide and the source and drain electrode layers
105a and 105b. In addition, an insulating film 107 functioning as a
base insulating film may be provided over the substrate 100 as
illustrated in FIG. 1C. The insulating film 107 can be formed with
a single layer or a stacked layer using a silicon nitride film, a
silicon oxynitride film, an aluminum oxide film, an aluminum
nitride film, an aluminum oxynitride film, or an aluminum nitride
oxide film.
The gate electrode layer 101 can be formed with a single layer or a
stacked layer using a metal material such as aluminum, copper,
molybdenum, titanium, chromium, tantalum, tungsten, neodymium, or
scandium; an alloy material which includes any of these materials
as its component; or a nitride which includes any of the materials
as a component. The gate electrode layer 101 is preferably formed
using a low-resistance conductive material such as aluminum or
copper; however, since the low-resistance conductive material has
disadvantages such as low heat resistance or a tendency to be
corroded, it is preferably used in combination with a conductive
material having heat resistance. As the heat-resistant conductive
material, molybdenum, titanium, chromium, tantalum, tungsten,
neodymium, scandium, or the like is used.
For example, as a stacked-layer structure of the gate electrode
layer 101, a two-layer structure in which a molybdenum layer is
stacked over an aluminum layer, a two-layer structure in which a
molybdenum layer is stacked over a copper layer, a two-layer
structure in which a titanium nitride layer or a tantalum nitride
layer is stacked over a copper layer, or a two-layer structure in
which a molybdenum layer and a titanium nitride layer are stacked
is preferable. Alternatively, a three-layer structure in which a
tungsten layer or a tungsten nitride layer, an aluminum-silicon
alloy layer or an aluminum-titanium alloy layer, and a titanium
nitride layer or a titanium layer are stacked is preferable.
The gate insulating layer 102 including silicon oxide is formed by
a plasma CVD method or a sputtering method. The gate insulating
layer 102 can be formed with a single layer or a stacked layer
using any of a silicon oxide layer, a silicon nitride layer, a
silicon oxynitride layer, and a silicon nitride oxide layer by a
CVD method, a sputtering method, or the like. Alternatively, the
gate insulating layer 102 can be formed using a silicon oxide layer
by a CVD method using an organosilane gas. The gate insulating
layer 102 may be formed with a single layer or a two or more
layers. For example, by forming the gate insulating film in contact
with the substrate 100, using a silicon nitride film or a silicon
nitride oxide film, adhesion between the substrate 100 and the gate
insulating layer is increased, and in the case where a glass
substrate is used as the substrate 100, an impurity can be
prevented from diffusing into the semiconductor layer from the
substrate and further, the gate electrode layer can be prevented
from being oxidized. That is to say, film peeling can be prevented,
and electric characteristics of the thin film transistor which is
formed later can be improved.
The oxide semiconductor layer 103 including silicon oxide can be
formed using a Zn--O-based non-single-crystal film, an
In--Ga--Zn--O-based non-single-crystal film, an
In--Sn--Zn--O-based, Ga--Sn--Zn--O-based, In--Zn--O-based,
Sn--Zn--O-based, In--Sn--O-based, or Ga--Zn--O-based oxide
semiconductor.
The oxide semiconductor layer 103 is formed using an oxide
semiconductor target including silicon oxide at 2.5 wt % to 20 wt %
inclusive, preferably at 7.5 wt % to 12.5 wt % inclusive. In this
embodiment, the oxide semiconductor layer 103 including silicon
oxide is formed by a sputtering method using an oxide semiconductor
target (ZnO) including silicon oxide at 10 wt %.
The source and drain regions 104a and 104b are preferably formed
using a degenerate oxide semiconductor. The degenerate oxide
semiconductor preferably transmits light. The source and drain
regions 104a and 104b are formed using an oxide semiconductor layer
which does not include silicon oxide, for example, a Zn--O-based
oxide semiconductor, an In--Ga--Zn--O-based oxide semiconductor, an
In--Zn--O-based oxide semiconductor, a Sn--Zn--O-based oxide
semiconductor, an In--Sn--O-based oxide semiconductor, an
Al--Zn--O-based oxide semiconductor, or a Ga--Zn--O-based oxide
semiconductor. Alternatively, the source and drain regions 104a and
104b may be formed using a Zn--O-based non-single-crystal film
including nitrogen, i.e., a Zn--O--N-based non-single-crystal film
(also referred to as a ZnON film); or an In--Ga--Zn--O-based
non-single-crystal film including nitrogen, i.e., an
In--Ga--Zn--O--N-based non-single-crystal film (also referred to as
an IGZON film). Further alternatively, the source and drain regions
104a and 104b may be formed using a Ga--Zn--O-based
non-single-crystal film; or a Ga--Zn--O-based non-single-crystal
film including nitrogen, i.e., a Ga--Zn--O--N-based
non-single-crystal film. Further alternatively, the source and
drain regions 104a and 104b may be formed from an Al--Zn--O-based
non-single-crystal film; or an Al--Zn--O-based non-single-crystal
film including nitrogen, i.e., an Al--Zn--O--N-based
non-single-crystal film. Note that each of the Al--Zn--O-based
oxide semiconductor and the Al--Zn--O--N-based oxide semiconductor
preferably includes aluminum at 1 wt % to 10 wt % inclusive, and
each of the Ga--Zn--O-based oxide semiconductor and the
Ga--Zn--O--N-based oxide semiconductor preferably includes gallium
at 1 wt % to 10 wt % inclusive. Further alternatively, a
Zn--O--N-based non-single-crystal film, which includes nitrogen, or
a Sn--Zn--O--N-based non-single-crystal film, which includes
nitrogen, may be used.
In this embodiment, the source and drain regions 104a and 104b are
formed using an oxynitride material. The oxynitride material is
obtained as follows: sputtering is performed in an atmosphere
including a nitrogen gas, with use of an oxide semiconductor target
(ZnO) including zinc (Zn), so that an oxynitride film including
zinc is formed; and the oxynitride film including zinc is subjected
to heat treatment.
The source and drain regions 104a and 104b do not include Si, which
is a major different point from the oxide semiconductor layer 103
including silicon oxide. In the source and drain regions 104a and
104b, in some cases, crystal grains are generated immediately after
the film formation or crystal grains are generated in the case
where heat treatment is performed after film formation. On the
other hand, in the oxide semiconductor layer 103 including silicon
oxide, the crystallization temperature of the film is high because
of inclusion of silicon oxide. Thus, for example, even when heat
treatment is performed at a temperature at which the source and
drain regions 104a and 104b are partly crystallized, the oxide
semiconductor layer 103 including silicon oxide can keep an
amorphous state. Note that the source and drain regions 104a and
104b are referred to as n.sup.+ regions or buffer layers.
In addition, in order to form an ohmic contact, the source or drain
region (a buffer layer) whose carrier concentration is higher than
that of the oxide semiconductor layer is intentionally formed
between the oxide semiconductor layer and the source electrode
layer (or the drain electrode layer). Note that the source and
drain regions have n-type conductivity and can be referred to as
n.sup.+ regions. In the case where the source and drain regions are
referred to as the n.sup.+ regions (N.sup.+-type regions), the
oxide semiconductor layer which is made to function as a channel
formation region can also be referred to as an i-type region
(1-type region) in contrast to the n.sup.+ regions. An NI junction
is formed by provision of the source and drain regions, so that a
semiconductor device provided with a thin film transistor having a
short channel length of 5 .mu.m or less and high field effect
mobility can be obtained.
The source and drain electrode layers 105a and 105b are formed
using any of an element selected from Al, Cr, Ta, Ti, Mo, and W, an
alloy including any of the elements as a component, an alloy film
including a combination of any of the elements, and the like.
Alternatively, indium tin oxide (ITO), indium tin oxide including
silicon oxide, aluminum doped zinc oxide (AZO), or gallium doped
zinc oxide (GZO) can be used. By adding an element to be a
trivalent ion such as Al.sub.2O.sub.3 or Ga.sub.2O.sub.3 to zinc
oxide by a small amount (for example, at a few wt %), the
resistance of the zinc oxide can be lowered.
The source and drain regions 104a and 104b enable contact
resistance with the source and drain electrode layers 105a and 105b
formed from a metal material with low electric resistance to be
reduced. Accordingly, by providing the source and drain regions
104a and 104b, the thin film transistor 160 with higher electric
characteristics is realized.
A protective insulating layer which covers and is in contact with
the oxide semiconductor layer 103 including silicon oxide and the
source and drain electrode layers 105a and 105b may be formed.
Further, the protective insulating layer can be formed with a
single layer or a stacked layer using any of a silicon nitride
film, a silicon oxide film, a silicon oxynitride film, and the
like, which is formed by a sputtering method or the like.
This embodiment describes an example of the thin film transistor
using the oxide semiconductor layer including silicon oxide.
Instead of the oxide semiconductor layer including silicon oxide,
an oxide semiconductor layer including silicon oxynitride may be
used.
Embodiment 2
This embodiment describes an example of a thin film transistor
which is different in the width of a gate electrode from that of
Embodiment 1 with reference to FIGS. 2A and 2B.
FIG. 2A illustrates a thin film transistor 170 which is one type of
a bottom-gate structure, and is an example of a cross-sectional
view of a structure called a channel-etch type. FIG. 2B is an
example of a top view of the thin film transistor whose cross
section taken along dotted line C1-C2 corresponds to FIG. 2A.
In the thin film transistor 170 illustrated in FIG. 2A, a gate
electrode layer 101 is provided over a substrate 100, a gate
insulating layer 102 is provided over the gate electrode layer 101,
an oxide semiconductor layer 103 is provided over the gate
insulating layer 102, and source and drain electrode layers 105a
and 105b are provided over the oxide semiconductor layer 103.
Between part of the oxide semiconductor layer 103 including silicon
oxide and the source and drain electrode layers 105a and 105b,
source and drain regions 104a and 104b are provided. Note that a
protective insulating layer covering the oxide semiconductor layer
103 and the source and drain electrode layers 105a and 105b may be
formed.
In this embodiment, over the gate insulating layer 102, the oxide
semiconductor layer 103 including oxide silicon (also referred to
as a first oxide semiconductor layer) and a second oxide
semiconductor layer (or an oxynitride layer) are stacked. Note that
because part of the second oxide semiconductor layer, which is
provided over a region which functions as a channel in the oxide
semiconductor layer 103 including silicon oxide is removed by
etching, the second oxide semiconductor layer is not provided over
the region. Note that the second oxide semiconductor layer (or the
oxynitride layer) functions as a buffer layer, an n.sup.+ region,
and source and drain regions. In FIG. 2A, the second oxide
semiconductor layer is illustrated as the source and drain regions
104a and 104b.
In this embodiment, the oxide semiconductor layer 103 including
silicon oxide is formed using an oxide semiconductor target
including zinc (Zn), in which silicon oxide (SiO.sub.2) is included
at 2.5 wt % to 20 wt % inclusive, preferably at 7.5 wt % to 12.5 wt
% inclusive. Inclusion of silicon oxide in an oxide semiconductor
facilitates amorphization of an oxide semiconductor film to be
formed. In addition, in the case where the oxide semiconductor film
is subjected to heat treatment, the oxide semiconductor film can be
prevented from being crystallized.
Structure Change caused by inclusion of SiO.sub.2 in an oxide
semiconductor including zinc (Zn), which is so-called ZnO, was
calculated by the classical molecular dynamics simulation. An
empirical potential which characterizes the interaction between
atoms is defined in the classical molecular dynamics method, so
that force that acts on each atom is evaluated. When Newton's
equation of motion is solved numerically by applying the classical
dynamic law on each atom, motion of atoms (time evolution) can be
deterministically tracked.
Calculation models and calculation conditions are described below.
Note that in this calculation, the Born-Mayer-Huggins potential was
used.
A calculation model is a ZnO single crystal structure of 896 atoms
(see FIG. 7). In this structure, Zn was replaced with Si and O.
Considering electric charge in each atom (Zn: +2, O: -2, Si: +4),
three Zn atoms were replaced with two Si atoms and one O atom. The
amount of replacement was defined as the following formula and
structures in the cases of the amounts of replacement of 2.5 wt %,
4.9 wt %, 7.6 wt %, 10.0 wt %, 12.5 wt %, 15.0 wt %, and 20.0 wt %
were formed. The structure in which Zn is replaced with Si and O is
referred to as a ZnO replacement structure.
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mes..times..times..times..times..times..times..times..times..times..times.-
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es..times..times..times..times..times..times..times..times..times..times..-
times..times..times..times..times. ##EQU00001##
At a temperature of 350.degree. C., structural relaxation was
performed at a fixed pressure (1 atm) for 400 psec (time step of
0.2 fsec.times.2000000 steps) with the classical molecular dynamics
simulation. Then, the radial distribution functions g(r) of the
eight structures were calculated. Note that the radial distribution
function g(r) is a function representing the probability density of
atoms existing at a distance of r from one atom. As the correlation
between atoms disappears, g(r) becomes closer to 1.
FIGS. 8A to 8E and FIGS. 9A to 9D show the radial distribution
functions g(r) of the calculation models, which are obtained by
performing the classical molecular dynamics simulation on the eight
calculation model for 400 psec.
In FIGS. 8A to 8E and FIGS. 9A to 9D, when the radial distribution
functions g(r) of the calculation models are compared with each
other, it can be found that in the case of the single crystal model
(see FIG. 8A), and in the cases where the amounts of replacement
are 2.5 wt % to 7.6 wt % (see FIGS. 8B to 8D), there is peak also
at a long distance, which shows existence of a long-range order.
When the amount of replacement is 10 wt % or more (see FIG. 8E, and
FIGS. 9A to 9D), there is no peaks at 0.6 nm or more, which shows
nonexistence of long-range order. From the above, it can be
considered that the model is made amorphous when the amount of
replacement is 10 wt % or more.
The classical molecular dynamics simulation was performed on the
eight calculation model for 400 psec, and final structures of the
calculation models were obtained. FIGS. 10A to 10E and FIGS. 11A to
11D show results obtained by simulation of XRD performed on the
final structures of the eight calculation models. Note that a
wavelength of X-ray for this calculation is 0.154138 nm (Cu Ka).
FIG. 10A shows a result of calculation of the ZnO single crystal
structure.
When the results of respective calculation models by simulation of
XRD are compared with each other in FIGS. 10A to 10E and FIGS. 11A
to 11D, it can be found that intensity of a peak of the amount of
replacement becomes weaker as the amount of replacement is
increased from 2.5 wt % (see FIG. 10B), as compared to the case of
the ZnO single crystal structure. Accordingly, it can be considered
that destruction of the single crystal structure as a whole and
amorphization begins at 2.5 wt % of the amount of replacement. In
addition, it is found that peaks are seen at 7.6 wt % or less of
the amount of replacement (see FIGS. 10C and 10D) and broad peaks
are seen at 10 wt % or more of the amount of replacement (FIG. 10E
and FIGS. 11A to 11D). Accordingly, amorphization is substantially
completed when the amount of replacement is 10 wt % or more.
The above calculation results suggest that inclusion of SiO.sub.2
in ZnO facilitates amorphization of ZnO. Actually, a ZnO thin film
including SiO.sub.2, which is obtained by a sputtering method is an
amorphous semiconductor film just after its formation. The above
calculation results show that inclusion of SiO.sub.2 interrupts
crystallization of ZnO even when heat treatment is performed, so
that an amorphous structure can be kept.
Instead of a Zn--O-based non-single-crystal film, the oxide
semiconductor layer 103 including silicon oxide can be formed using
an In--Ga--Zn--O-based non-single-crystal film, an
In--Sn--Zn--O-based, Ga--Sn--Zn--O-based, In--Zn--O-based,
Sn--Zn--O-based, In--Sn--O-based, or Ga--Zn--O-based oxide
semiconductor.
In addition, the source and drain regions 104a and 104b are
preferably formed using a degenerate oxide semiconductor. The
degenerate oxide semiconductor preferably transmits light. Further,
as the source and drain regions 104a and 104b, an oxide
semiconductor layer which does not include silicon oxide, for
example, a Zn--O-based oxide semiconductor, an In--Ga--Zn--O-based
oxide semiconductor, an In--Zn--O-based oxide semiconductor, a
Sn--Zn--O-based oxide semiconductor, an In--Sn--O-based oxide
semiconductor, an Al--Zn--O-based oxide semiconductor, or a
Ga--Zn--O-based oxide semiconductor may be used. Alternatively, the
source and drain regions 104a and 104b may be formed using an
In--Ga--Zn--O-based non-single-crystal film including nitrogen,
that is, an In--Ga--Zn--O--N-based non-single-crystal film (also
referred to as an IGZON film). Further alternatively, the source
and drain regions 104a and 104b may be formed using a
Ga--Zn--O-based non-single-crystal film; or a Ga--Zn--O-based
non-single-crystal film including nitrogen, that is, a
Ga--Zn--O--N-based non-single-crystal film. Further alternatively,
the source and drain regions 104a and 104b may be formed using an
Al--Zn--O-based non-single-crystal film; or an Al--Zn--O-based
non-single-crystal film including nitrogen, that is, an
Al--Zn--O--N-based non-single-crystal film. Note that each of the
Al--Zn--O-based oxide semiconductor and the Al--Zn--O--N-based
oxide semiconductor preferably includes aluminum at 1 wt % to 10 wt
% inclusive, and each of the Ga--Zn--O-based oxide semiconductor
and the Ga--Zn--O--N-based oxide semiconductor preferably includes
gallium at 1 wt % to 10 wt % inclusive. Further alternatively, a
Zn--O--N-based non-single-crystal film, which includes nitrogen, or
a Sn--Zn--O--N-based non-single-crystal film, which includes
nitrogen, may be used.
In this embodiment, the source and drain regions 104a and 104b are
formed using an oxynitride material. The oxynitride material is
obtained as follows: sputtering is performed in an atmosphere
including a nitrogen gas using an oxide semiconductor target (ZnO)
including zinc (Zn), so that a Zn--O--N-based non-single-crystal
film is formed; and the Zn--O--N-based non-single-crystal film is
subjected to heat treatment.
The source and drain electrode layers 105a and 105b are formed
using any of an element selected from Al, Cr, Ta, Ti, Mo, and W, an
alloy including any of the elements as a component, an alloy film
including a combination of any of the elements, and the like.
Alternatively, indium tin oxide (ITO), indium tin oxide including
silicon oxide (SiO.sub.X), aluminum doped zinc oxide (AZO), or
gallium doped zinc oxide (GZO) can be used.
An example of manufacturing a display device using the above thin
film transistor 170 as a switching element of a pixel portion is
described below with reference to FIG. 3.
First, the gate electrode layer 101 is provided over the substrate
100 having an insulating surface. A glass substrate is used as the
substrate 100 having an insulating surface. The gate electrode
layer 101 can be formed with a single layer or a stacked layer
using a metal material such as molybdenum, titanium, chromium,
tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an
alloy material which includes any of the materials as a main
component. Note that when the gate electrode layer 101 is formed, a
capacitor wiring 108 of the pixel portion and a first terminal 121
of a terminal portion are formed together. Note that an insulating
film functioning as a base insulating film may be formed over the
substrate 100. The insulating film can also be formed with a single
layer or a stacked layer using a silicon nitride film, a silicon
oxynitride film, an aluminum oxide film, an aluminum nitride film,
an aluminum oxynitride film, or an aluminum nitride oxide film
For example, as a two-layer structure of the gate electrode layer
101, the following structures are preferable: a two-layer structure
in which a molybdenum layer is stacked over an aluminum layer, a
two-layer structure in which a molybdenum layer is stacked over a
copper layer, a two-layer structure in which a titanium nitride
layer or a tantalum nitride layer is stacked over a copper layer,
and a two-layer structure in which a titanium nitride layer and a
molybdenum layer are stacked. Alternatively, a stack including a
copper layer including Ca and a copper oxide layer including Ca
thereover, which serves as a barrier layer, or a stack including a
copper layer including Mg and a copper oxide layer including Mg
thereover, which serves as a barrier layer, can be employed. As a
three-layer structure, a stack of a tungsten layer or a tungsten
nitride layer, a layer of an alloy of aluminum and silicon or an
alloy of aluminum and titanium, and a titanium nitride layer or a
titanium layer is preferable.
Next, the gate insulating layer 102 covering the gate electrode
layer 101 is formed. The gate insulating layer 102 is formed to a
thickness of 50 nm to 400 nm by a sputtering method, a PCVD method,
or the like.
For example, as the gate insulating layer 102, a silicon oxide film
is formed to a thickness of 100 nm by a sputtering method. Needless
to say, the gate insulating layer 102 is not limited to such a
silicon oxide film, and other insulating films such as a silicon
oxynitride film, a silicon nitride film, an aluminum oxide film, an
aluminum nitride film, an aluminum oxynitride film, or a tantalum
oxide film may be used to form a single-layer structure or a
stacked-layer structure. In a case of forming a stack of layers,
for example, a silicon nitride film may be formed by a PCVD method
and then a silicon oxide film may be formed thereover by a
sputtering method. When a silicon oxynitride film, a silicon
nitride film, or the like is used as the gate insulating layer 102,
an impurity from the glass substrate, sodium for example, can be
blocked from diffusing into and entering an oxide semiconductor to
be formed thereover later.
Next, an oxide semiconductor film including silicon oxide is formed
over the gate insulating layer 102. Here, formation is performed
with use of an oxide semiconductor target including zinc (Zn), in
which silicon oxide (SiO.sub.2) is included at 10 wt %. Inclusion
of silicon oxide in an oxide semiconductor facilitates
amorphization of an oxide semiconductor film to be formed. In
addition, when heat treatment is performed in a process after
formation of the oxide semiconductor film, the oxide semiconductor
film including silicon oxide can be prevented from being
crystallized.
Then, an oxynitride film which does not include silicon oxide is
formed over the oxide semiconductor film including silicon oxide by
a sputtering method. Here, sputtering is performed in an atmosphere
including a nitrogen gas using an oxide semiconductor target (ZnO)
including zinc (Zn), so that a Zn--O--N-based non-single-crystal
film is formed.
Examples of a sputtering method include an RF sputtering method in
which a high-frequency power source is used as a sputtering power
source, a DC sputtering method, and a pulsed DC sputtering method
in which a bias is applied in a pulsed manner.
In addition, there is also a multi-source sputtering apparatus in
which a plurality of targets of different materials can be set.
With the multi-source sputtering apparatus, films of different
materials can be formed to be stacked in the same chamber, or a
film of plural kinds of materials can be formed by electric
discharge at the same time in the same chamber.
In addition, there are a sputtering apparatus provided with a
magnet system inside the chamber and used for a magnetron
sputtering, and a sputtering apparatus used for an ECR sputtering
in which plasma generated with the use of microwaves is used
without using glow discharge.
Furthermore, as a deposition method by sputtering, there are also a
reactive sputtering method in which a target substance and a
sputtering gas component are chemically reacted with each other
during deposition to form a thin compound film thereof, and a bias
sputtering in which a voltage is also applied to a substrate during
deposition.
Next, a photolithography step is performed. A resist mask is
formed, and the Zn--O--N-based non-single-crystal film (the
oxynitride film) is selectively etched. Then, with use of the same
mask, the Zn--O-based non-single-crystal film (the oxide
semiconductor film) including silicon oxide is selectively etched.
The resist mask is removed after the etching.
Next, a photolithography step is performed and a new resist mask is
formed. Unnecessary portions (part of the gate insulating layer)
are removed by etching, so that a contact hole reaching an
electrode layer or a wiring formed from the same material as the
gate electrode layer is formed. The contact hole is provided for
direct connection with a conductive film to be formed later. For
example, a contact hole is formed when a thin film transistor whose
gate electrode layer is in direct contact with the source or drain
electrode layer in the driver circuit portion is formed, or when a
terminal that is electrically connected to a gate wiring of a
terminal portion is formed. Note that an example in which a contact
hole is formed by a photolithography step for direct connection
with the conductive film to be formed later is described here
without particular limitation. The contact hole reaching the gate
electrode layer may be formed later in a step of forming a contact
hole for connection with a pixel electrode and the same material as
the pixel electrode may be used for electrical connection. In the
case where electrical connection is performed with use of the same
material as the pixel electrode, one mask can be reduced.
Then, a conductive film formed from a metal material is formed over
the Zn--O--N-based non-single-crystal film (the oxynitride film) by
a sputtering method or a vacuum evaporation method.
As a material for the conductive film, an element selected from Al,
Cr, Ta, Ti, Mo, and W; an alloy including any of the elements as a
component; an alloy film including a combination of any of the
elements; and the like can be given. Further, in the case of
performing heat treatment at 200.degree. C. to 600.degree. C.
later, the conductive film preferably has heat resistance against
such heat treatment. Further, for heat treatment at 200.degree. C.
to 600.degree. C., the conductive film preferably has heat
resistance for such heat treatment. Since use of Al alone brings
disadvantages such as low heat resistance and a tendency to be
corroded, aluminum is used in combination with a conductive
material having heat resistance. As the conductive material having
heat resistance which is used in combination with Al, any of the
following materials may be used: an element selected from titanium
(Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr),
and neodymium (Nd), scandium (Sc), an alloy including any of the
elements as a component, an alloy film including the elements in
combination, and a nitride including any of the elements as a
component. Alternatively, indium tin oxide (ITO), indium tin oxide
including silicon oxide (SiO.sub.X), zinc oxide including aluminum
(AZO), or zinc oxide including gallium (GZO) can also be used. By
adding an element to be a trivalent ion such as Al.sub.2O.sub.3 or
Ga.sub.2O.sub.3 to zinc oxide by a small amount (for example, at a
few wt %), the resistance of the zinc oxide can be lowered.
In this embodiment, the conductive film has a single-layer
structure of a titanium film. Further, the conductive film may have
a two-layer structure, and a titanium film may be stacked over an
aluminum film. Still alternatively, the conductive film may have a
three-layer structure including a Ti film, an aluminum film
including Nd (Al--Nd) which is stacked on the Ti film, and a Ti
film formed on these films. The conductive film may have a
single-layer structure of an aluminum film including silicon.
Next, a photolithography step is performed and a resist mask is
formed. Then, unnecessary portions are removed by etching, so that
the source and drain electrode layers 105a and 105b and the source
and drain regions 104a and 104b are formed in the pixel portion and
source and drain electrode layers and source and drain regions are
formed in the driver circuit portion. Wet etching or dry etching is
employed as an etching method at this time. For example, when an
aluminum film or an aluminum-alloy film is used as the conductive
film, wet etching using a mixed solution of phosphoric acid, acetic
acid, and nitric acid can be carried out. Here, the conductive film
of the Ti film is etched by wet etching to form the source and
drain electrode layers, and the Zn--O--N-based non-single-crystal
film is etched to form a first buffer layer (the source or drain
region 104a) and a second buffer layer (the source or drain region
104b). In this etching step, an exposed region of the oxide
semiconductor film including silicon oxide is also partly etched,
so that the oxide semiconductor layer 103 including silicon oxide
is formed.
In the photolithography step, a second terminal 122 formed from the
same material as the source and drain electrode layers 105a and
105b remains in the terminal portion. Note that the second terminal
122 is electrically connected to a source wiring (the source wiring
including the source and drain electrode layers 105a and 105b).
Through the above steps, the thin film transistor 170 in which the
oxide semiconductor layer 103 including silicon oxide serves as a
channel formation region can be formed in the pixel portion.
In the terminal portion, the connection electrode 120 is directly
connected to the first terminal 121 in the terminal portion through
the contact hole formed in the gate insulating film. Note that
although not illustrated in this embodiment, a source or drain
wiring of the thin film transistor of the driver circuit is
directly connected to the gate electrode through the same steps as
the above steps.
Next, heat treatment is preferably performed at 200.degree. C. to
600.degree. C., typically 300.degree. C. to 500.degree. C. (the
heat treatment may be annealing with light). Here, heat treatment
is performed in a nitrogen atmosphere in a furnace at 350.degree.
C. for 1 hour. Through this heat treatment, rearrangement at the
atomic level occurs in the Zn--O-based non-single-crystal film
including silicon oxide. In addition, the oxide semiconductor layer
103 including silicon oxide can be prevented from being
crystallized in heat treatment because of inclusion of silicon
oxide; thus the oxide semiconductor layer 103 can keep an amorphous
structure. Note that there is no particular limitation on when to
perform the heat treatment, as long as it is performed after the
Zn--O--N-based non-single-crystal film is formed. For example, the
heat treatment may be performed after a pixel electrode is
formed.
Next, the resist mask is removed, and a protective insulating layer
106 is formed to cover the thin film transistor 170.
Next, a photolithography step is performed and a resist mask is
formed. Then, the protective insulating layer 106 is etched, so
that a contact hole reaching the source or drain electrode layer
105b is formed. In addition, by the etching here, a contact hole
reaching the second terminal 122 and a contact hole reaching the
connection electrode 120 are formed.
Next, the resist mask is removed, and then a transparent conductive
film is formed. The transparent conductive film is formed using
indium oxide (In.sub.2O.sub.3), indium tin oxide
(In.sub.2O.sub.3-SaO.sub.2, abbreviated as ITO), or the like by a
sputtering method, a vacuum evaporation method, or the like. Such a
material is etched with a hydrochloric acid-based solution.
However, since etching of ITO especially tends to leave residue, an
alloy of indium oxide and zinc oxide (In.sub.2O.sub.3--ZnO) may be
used in order to improve etching processability. AZO or GZO may be
used.
Next, a photolithography step is performed and a resist mask is
formed. Then, unnecessary portions are removed by etching, so that
a pixel electrode layer 110 is formed. In this photolithography
step, a storage capacitor is formed by the capacitor wiring 108 and
the pixel electrode layer 110 using the gate insulating layer 102
and the protective insulating layer 106 in the capacitor portion as
dielectrics. In addition, in this photolithography step, the first
terminal and the second terminal are covered with the resist mask,
and transparent conductive films 128 and 129 are left in the
terminal portion. The transparent conductive films 128 and 129
function as electrodes or wirings connected to an FPC. The
transparent conductive film 128 formed over the connection
electrode 120 which is directly connected to the first terminal 121
is a connection terminal electrode which functions as an input
terminal of the gate wiring. The transparent conductive film 129
formed over the second terminal 122 is a connection terminal
electrode which functions as an input terminal of the source
wiring.
This embodiment describes an example in which the storage capacitor
is formed by the capacitor wiring 108 and the pixel electrode layer
110 by using the gate insulating layer 102 and the protective
insulating layer 106 as dielectrics is described; however, there is
no particular limitation. A structure may also be employed, in
which an electrode formed using the same material as the source and
drain electrodes is provided above a capacitor wiring and a storage
capacitor is formed by the electrode and the capacitor wiring with
the gate insulating layer 102 interposed therebetween as a
dielectric, and the electrode and the pixel electrode layer 110 are
electrically connected.
Then, the resist mask is removed. A cross-sectional view at this
stage is illustrated in FIG. 3. Note that, a top view of the thin
film transistor 170 in the pixel portion at this stage corresponds
to FIG. 4.
A cross-sectional view taken along line A1-A2 and line B1-B2 of
FIG. 4 corresponds to FIG. 3. FIG. 3 illustrates a cross-sectional
structure of the thin film transistor 170 in the pixel portion, a
cross-sectional structure of a capacitor portion in the pixel
portion, and a cross-sectional structure of the terminal
portion.
FIGS. 5A-1 and 5A-2 respectively illustrate a cross-sectional view
and a top view of a gate wiring terminal portion at this stage.
FIG. 5A-1 is a cross-sectional view taken along line C1-C2 of FIG.
5A-2. In FIG. 5A-1, a transparent conductive film 155 formed over
the protective insulating layer 106 is a connection terminal
electrode which functions as an input terminal. In the terminal
portion in FIG. 5A-1, a first terminal 151 formed using the same
material as the material of the gate wiring and a connection
electrode 153 formed using the same material as the material of the
source wiring overlap each other with a gate insulating layer 152
interposed therebetween and are electrically connected through the
transparent conductive film 155.
FIGS. 5B-1 and 5B-2 are a cross-sectional view and a top view of a
source wiring terminal portion, respectively. FIG. 5B-1 corresponds
to the cross-sectional view taken along line D1-D2 in FIG. 5B-2. In
FIG. 5B-1, a transparent conductive film 155 formed over the
protective insulating film 106 is a connection terminal electrode
functioning as an input terminal. Furthermore, in FIG. 5B-1, in the
terminal portion, an electrode 156 formed from the same material as
the gate wiring is located below and overlapped with a second
terminal 150, which is electrically connected to the source wiring,
with the gate insulating layer 152 interposed therebetween. The
electrode 156 is not electrically connected to the second terminal
150, and a capacitor to prevent noise or static electricity can be
formed when the potential of the electrode 156 is set to a
potential different from that of the second terminal 150, such as
floating, GND, or 0 V. The second terminal 150 is electrically
connected to the transparent conductive film 155 with the
protective insulating layer 106 interposed therebetween.
A plurality of gate wirings, source wirings, and capacitor wirings
are provided depending on the pixel density. Also in the terminal
portion, the first terminal at the same potential as the gate
wiring, the second terminal at the same potential as the source
wiring, the third terminal at the same potential as the capacitor
wiring, and the like are each arranged in plurality. The number of
each of the terminals may be any number, and the number of the
terminals may be determined by a practitioner as appropriate.
Thus, the pixel portion which includes the thin film transistor 170
including an oxide semiconductor layer including silicon oxide and
the storage capacitor, and the terminal portion can be
manufactured. In addition, a driver circuit can be formed over the
same substrate.
In the case of manufacturing an active matrix liquid crystal
display device, an active matrix substrate and a counter substrate
provided with a counter electrode are bonded to each other with a
liquid crystal layer interposed therebetween. Note that a common
electrode electrically connected to the counter electrode on the
counter substrate is provided over the active matrix substrate, and
a terminal electrically connected to the common electrode is
provided in the terminal portion. This terminal is provided so that
the common electrode is fixed to a predetermined potential such as
GND or 0 V.
Further, in this embodiment, a pixel structure is not limited to
that of FIG. 4. An example of a top view different from FIG. 4 is
illustrated in FIG. 6. FIG. 6 illustrates an example in which a
capacitor wiring is not provided but a protective insulating film
and a gate insulating layer are sandwiched between a pixel
electrode and a gate wiring of an adjacent pixel to form a storage
capacitor. In that case, a capacitor wiring and a third terminal
which is connected to the capacitor wiring can be omitted. Note
that in FIG. 6, the same portions as those in FIG. 4 are denoted by
the same reference numerals.
In an active matrix liquid crystal display device, pixel electrodes
arranged in a matrix form are driven to form a display pattern on a
screen. Specifically, voltage is applied between a selected pixel
electrode and a counter electrode corresponding to the pixel
electrode, so that a liquid crystal layer provided between the
pixel electrode and the counter electrode is optically modulated
and this optical modulation is recognized as a display pattern by
an observer.
In displaying moving images, a liquid crystal display device has a
problem that a long response time of liquid crystal molecules
themselves causes afterimages or blurring of moving images. In
order to improve the moving-image characteristics of a liquid
crystal display device, a driving method called black insertion is
employed in which black is displayed on the whole screen every
other frame period.
Alternatively, a driving method called double-frame rate driving
may be employed in which a vertical synchronizing frequency is 1.5
times or more, preferably, 2 times or more as high as a usual
vertical synchronizing frequency, whereby the moving-image
characteristics are improved.
Further alternatively, in order to improve the moving-image
characteristics of a liquid crystal display device, a driving
method may be employed, in which a plurality of LEDs
(light-emitting diodes) or a plurality of EL light sources are used
to form a surface light source as a backlight, and each light
source of the surface light source is independently driven in a
pulsed manner in one frame period. As the surface light source,
three or more kinds of LEDs may be used and an LED emitting white
light may be used. Since a plurality of LEDs can be controlled
independently, the light emission timing of LEDs can be
synchronized with the timing at which a liquid crystal layer is
optically modulated. According to this driving method, LEDs can be
partly turned off; therefore, an effect of reducing power
consumption can be obtained particularly in the case of displaying
an image having a large part on which black is displayed.
By combining these driving methods, the display characteristics of
a liquid crystal display device, such as moving-image
characteristics, can be improved as compared to those of
conventional liquid crystal display devices.
In addition, according to this embodiment, a display device having
high electrical properties and high reliability can be provided at
low costs.
This embodiment mode can be arbitrarily combined with Embodiment
Mode 1.
Embodiment 3
This embodiment describes an example in which exposure using a
multi-tone mask is performed in order to reduce the number of
masks.
In addition, this embodiment describes an example in which indium
that is a rare metal the amount of production of which is limited
is not used in the composition of an oxide semiconductor layer. In
addition, this embodiment describes an example in which gallium
that is one kind of rare metal is also not used as a compositional
element of an oxide semiconductor layer.
Note that a multi-tone mask can perform three levels of light
exposure to obtain an exposed portion, a half-exposed portion, and
an unexposed portion. Light has a plurality of intensity levels
after passing through a multi-tone mask. One-time light exposure
and development process can form a resist mask with regions of
plural thicknesses (typically, two kinds of thicknesses) to be
formed. Accordingly, by using a multi-tone mask, the number of
photomasks can be reduced.
As typical examples of a multi-tone mask, there are a gray-tone
mask, a half-tone mask, and the like.
A gray-tone mask includes a substrate having a light-transmitting
property, and a light-blocking portion and a diffraction grating
which are formed thereover. The light transmittance of the
light-shielding portion is 0%. In contrast, the light transmittance
of the diffraction grating can be controlled by setting an interval
between light-transmitting portions in slit forms, dot forms, or
mesh forms to an interval less than or equal to the resolution
limit of light used for the exposure. Note that the diffraction
grating can be in a regular slit form, a regular dot form, or a
regular mesh form, or in an irregular slit form, an irregular dot
form, or an irregular mesh form.
A half-tone mask includes a substrate having a light-transmitting
property, and a semi-light-transmitting portion and a
light-blocking portion which are formed thereover. The
transflective portion can be formed using MoSiN, MoSi, MoSiO,
MoSiON, CrSi, or the like. The light-shielding portion can be
formed using a light-shielding material which absorbs light, such
as chromium or chromium oxide. When the half-tone mask is
irradiated with light for exposure, the light transmittance of the
light-blocking portion is 0% and the light transmittance of a
region where the light-blocking portion and the
semi-light-transmitting portion are not provided is 100%. The light
transmittance of the semi-light-transmitting portion can be
controlled in the range of from 10% to 70%. The light transmittance
of the transflective portion can be controlled by controlling the
material used for the transflective portion.
FIGS. 12A to 12E correspond to cross-sectional views illustrating
steps for manufacturing a thin film transistor 360.
In FIG. 12A, an insulating film 357 is provided over a substrate
350 and a gate electrode layer 351 is provided thereover. In this
embodiment, a silicon oxide film (having a thickness of 100 nm) is
used as the insulating film 357. Over the gate electrode layer 351,
a gate insulating layer 352, an oxide semiconductor film 380
including silicon oxide, an oxynitride film 381, and a conductive
film 383 are stacked in this order. In this embodiment, an oxide
semiconductor which does not include indium and gallium, typically
a Zn--O-based oxide semiconductor, or a Sn--Zn--O-based oxide
semiconductor is used as the oxide semiconductor film 380 including
silicon oxide. In this embodiment, a Zn--O-based oxide
semiconductor formed by a sputtering method is used as the oxide
semiconductor film 380 including silicon oxide. In addition, a
Zn--O--N-based oxynitride material which does not include silicon
oxide is used as the oxynitride film 381.
Next, a mask 384 is formed over the gate insulating layer 352, the
oxide semiconductor film 380 including silicon oxide, the
oxynitride film 381, and the conductive film 383.
This embodiment describes an example in which light exposure using
a multi-tone (high-tone) mask is performed for forming the mask
384.
The light exposure is performed using the multi-tone mask through
which light has a plurality of intensity levels, and then
development is performed, whereby the mask 384 having regions with
different levels of thickness can be formed as shown in FIG. 12B.
The number of light-exposure masks can be reduced by using a
multi-tone mask.
Next, a first etching step is performed using the mask 384 and the
oxide semiconductor film 380 including silicon oxide, the
oxynitride film 381, and the conductive film 383 are etched into an
island shape. Accordingly, a patterned oxide semiconductor layer
390 including silicon oxide, a patterned oxynitride layer 385, and
a patterned conductive layer 387 can be formed (see FIG. 12B).
Then, the resist mask 384 is subjected to ashing. As a result, the
area and thickness of the mask are reduced. At this time, the
resist of the mask in a region with a small thickness (a region
overlapping with part of the gate electrode layer 351) is removed,
and divided masks 388 can be formed (see FIG. 12C).
A second etching step is performed using the masks 388, and the
oxynitride layer 385 and the conductive layer 387 are etched, so
that an oxide semiconductor layer 353 including silicon oxide,
source and drain regions 354a and 354b, and source and drain
electrode layers 355a and 355b are formed (see FIG. 12D). Note that
the oxide semiconductor layer 353 including silicon oxide is partly
etched to become an oxide semiconductor layer having a groove
(depression) and also having an end portion which is partly etched
and exposed to outside.
When the first etching step is performed on the oxynitride film 381
and the conductive film 383 by dry etching, the oxynitride film 381
and the conductive film 383 are etched anisotropically, which makes
the end portions of the mask 384 and the end portions of the
oxynitride layer 385 and the conductive layer 387 to be aligned
with each other so as to become continuous.
Similarly, when the second etching step is performed on the
oxynitride layer 385 and the conductive layer 387 by dry etching,
the oxynitride layer 385 and the conductive layer 387 are etched
anisotropically, which makes the end portions of the masks 388, an
end portion of the depression, end portions in the etched region of
the oxide semiconductor layer 353 including silicon oxide, end
portions of the source and drain regions 354a and 354b, and end
portions of the source and drain electrode layers 355a and 355b to
be aligned with each other so as to become continuous.
This embodiment describes the case where the oxide semiconductor
layer 353 including silicon oxide and the source and drain
electrode layers 355a and 355b have the same tapered angle at the
respective end portions and are stacked so that the end portions
are continuous. However, since the etching rates thereof are
different depending on the etching condition and the materials of
the oxide semiconductor layer and the conductive layer, the tapered
angles may be different and the end portions is not necessarily
continuous.
Then, the mask 388 is removed.
Next, heating is performed at 200.degree. C. to 600.degree. C. in
an atmosphere including oxygen (see FIG. 12E). The oxide
semiconductor layer 353 includes silicon oxide which interrupts
crystallization; thus, the oxide semiconductor layer including
silicon oxide can keep the amorphous state even after heating at
200.degree. C. to 600.degree. C.
Through the above process, the channel-etch type thin film
transistor 360 in which the oxide semiconductor layer 353 including
silicon oxide is provided can be manufactured.
The use of a resist mask having regions of plural thicknesses
(typically, two kinds of thicknesses) formed with use of a
multi-tone mask as in this embodiment enables the number of resist
masks to be reduced; therefore, the process can be simplified and
cost can be reduced.
Further, indium and gallium are not used in the oxide semiconductor
layer including silicon oxide or the oxynitride layer as described
in this embodiment, thereby reducing the cost for a target of an
oxide semiconductor, which leads to cost reduction.
Accordingly, a semiconductor device can be manufactured at low cost
with high productivity.
Embodiment 4
This embodiment describes an example of a channel stop type thin
film transistor 430 using FIGS. 13A and 13B. FIG. 13B illustrates
an example of a top view of a thin film transistor, cross-sectional
view taken along dotted line Z1-Z2 of which corresponds to FIG.
13A. Described is an example in which an oxide semiconductor
material which does not include indium is used in an oxide
semiconductor layer in the thin film transistor 430.
In FIG. 13A, a gate electrode 401 is provided over a substrate 400.
Then, over a gate insulating layer 402 covering the gate electrode
401, an oxide semiconductor layer 403 including silicon oxide is
provided.
In this embodiment, a Zn--O-based oxide semiconductor formed by a
sputtering method is used as the oxide semiconductor layer 403
including silicon oxide. In this embodiment, the oxide
semiconductor layer 403 including silicon oxide is formed using an
oxide semiconductor which does not include indium, typically, a
Zn--O-based, Ga--Sn--Zn--O-based, Ga--Zn--O-based, Sn--Zn--O-based,
or Ga--Sn--O-based oxide semiconductor which does not include
indium.
Next, a channel protective layer 418 is provided in contact with
and over the oxide semiconductor layer 403 including silicon oxide.
The channel protective layer 418 is provided, so that damage to a
channel formation region of the oxide semiconductor layer 403
including silicon oxide (for example, reduction in thickness due to
plasma or an etchant in etching, or oxidation) can be prevented in
the manufacturing process. Accordingly, the reliability of the thin
film transistor 430 can be improved.
The channel protective layer 418 can be formed using an inorganic
material (such as silicon oxide, silicon nitride, silicon
oxynitride, or silicon nitride oxide). As a method for forming the
channel protective layer 418, a vapor deposition method such as a
plasma CVD method or a thermal CVD method or a sputtering method
can be used. After the formation of the channel protective layer
418, the shape thereof is processed by etching. In this embodiment,
a silicon oxide film is formed by a sputtering method and processed
by etching using a mask formed by photolithography, so that the
channel protective layer 418 is formed.
Next, source and drain regions 406a and 406b are formed over the
channel protective layer 418 and the oxide semiconductor layer 403
including silicon oxide. In this embodiment, the source and drain
regions 406a and 406b are formed using a Ga--Zn--O--N-based
non-single-crystal film is used. Alternatively, the source and
drain regions 406a and 406b may be formed using a Zn--O-based
non-single-crystal film including nitrogen, i.e., a Zn--O--N
non-single-crystal film.
Next, a first wiring 409 is formed over the source or drain region
406a and a second wiring 410 is formed over the source or drain
region 406b. The first wiring 409 and the second wiring 410 are
formed using an element selected from Al, Cr, Ta, Ti, Mo, and W, an
alloy including any of the elements as a component, an alloy film
including a combination of any of the elements, or the like.
Alternatively, indium tin oxide (ITO), zinc oxide including
aluminum (AZO: aluminum doped zinc oxide), or zinc oxide including
gallium (GZO: gallium doped zinc oxide) can be used. By adding an
element to be a trivalent ion such as Al.sub.2O.sub.3 or
Ga.sub.2O.sub.3 to zinc oxide by a small amount (for example, at a
few wt %), the resistance of the zinc oxide can be lowered.
By the provision of the source and drain regions 406a and 406b, the
first wiring 409 and the second wiring 410 which are metal layers
can have a good junction with the oxide semiconductor layer 403
including silicon oxide, so that stable operation can be performed
in terms of heat in comparison with a Schottky junction. In
addition, it is effective that the source and drain regions 406a
and 406b are provided in order that carriers of a channel are
supplied (on the source side), that carriers of a channel are
absorbed stably (on the drain side), or that a resistive component
is not produced in an interface between a wiring and an oxide
semiconductor layer.
Next, heat treatment is preferably performed at 200.degree. C. to
600.degree. C., typically, 300.degree. C. to 500.degree. C. Here,
heat treatment is performed in air in a furnace at 350.degree. C.
for 1 hour. Through this heat treatment, rearrangement at the
atomic level occurs in the oxide semiconductor layer 403 including
silicon oxide. Because strain which inhibits carrier movement is
released by the heat treatment, the heat treatment (the heat
treatment may annealing with light) is important. In addition,
silicon oxide included in the oxide semiconductor layer 403 can
prevent the oxide semiconductor layer 403 from being crystallized
in heat treatment; thus, a large part of the oxide semiconductor
layer 403 can keep the amorphous structure. There is no particular
limitation on when to perform the heat treatment as long as it is
performed after the formation of the oxide semiconductor layer 403
including silicon oxide; for example, it can be performed after the
formation of a pixel electrode.
Further, indium is not used in the oxide semiconductor layer as is
in this embodiment, which leads to no use of indium that might be
depleted as a material.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 5
This embodiment describes an example in which an inverter circuit
is formed using two n-channel thin film transistors 760 and 761
with reference to FIGS. 14A and 14B. This embodiment describes an
example in which gallium is not included in each oxide
semiconductor layer of the thin film transistors 760 and 761.
The driver circuit for driving a pixel portion is formed using an
inverter circuit, a capacitor, a resistor, and the like. When the
inverter circuit is formed using two n-channel TFTs in combination,
there are an inverter circuit having a combination of an
enhancement type transistor and a depletion type transistor
(hereinafter, referred to as an EDMOS circuit) and an inverter
circuit having a combination of two enhancement type TFTs
(hereinafter, referred to as an EEMOS circuit). Note that an
n-channel TFT whose threshold voltage is positive is referred to as
an enhancement type transistor, and an n-channel TFT whose
threshold voltage is negative is referred to as a depletion type
transistor, throughout this specification.
The pixel portion and the driver circuit are formed over one
substrate. In the pixel portion, on and off of voltage application
to a pixel electrode are switched using enhancement type
transistors arranged in a matrix.
FIG. 14A illustrates a cross-sectional structure of the inverter
circuit of the driver circuit. In FIG. 14A, a first gate electrode
741 and a second gate electrode 742 are provided over a substrate
740. The first gate electrode 741 and the second gate electrode 742
each can be formed with a single layer or a stacked layer using any
of a metal material such as molybdenum, titanium, chromium,
tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an
alloy material including any of the materials as a main
component.
Further, a first wiring 749, a second wiring 750, and a third
wiring 751 are provided over a gate insulating layer 743 that
covers the first gate electrode 741 and the second gate electrode
742. The second wiring 750 is directly connected to the second gate
electrode 742 through a contact hole 744 formed in the gate
insulating layer 743.
Further, a source or drain region 755a is formed over the first
wiring 749, a source or drain region 755b and a source or drain
region 756a are formed over the second wiring 750, and a source or
drain region 756b is formed over the third wiring 751. In this
embodiment, the source and drain regions 755a and 755b and the
source and drain regions 756a and 756b are formed using a
Zn--O--N-based non-single-crystal film which does not include
silicon oxide. Alternatively, the source and drain regions 755a and
755b and the source and drain regions 756a and 756b may be formed
using an In--Zn--O--N-based non-single-crystal film including
nitrogen may be used.
Further, a first oxide semiconductor layer 745 including silicon
oxide is provided in a position which overlaps with the first gate
electrode 741 and which is over the first and second wirings 749
and 750 with the source and drain regions to 755a and 755b
interposed therebetween. A second oxide semiconductor layer 747
including silicon oxide is provided in a position which overlaps
with the second gate electrode 742 and which is over the second and
third wirings 750 and 751 with the source and drain regions 756a
and 756b interposed therebetween.
In this embodiment, the first oxide semiconductor layer 745
including silicon oxide and the second oxide semiconductor layer
747 including silicon oxide are formed using a Zn--O-based oxide
semiconductor by a sputtering method. For the first oxide
semiconductor layer 745 including silicon oxide and the second
oxide semiconductor layer 747 including silicon oxide, an oxide
semiconductor which does not contain gallium, typically, an
In--Sn--Zn--O-based, In--Zn--O-based, In--Sn--O-based,
Sn--Zn--O-based, or Zn--O-based oxide semiconductor, which does not
include gallium, is used.
The first thin film transistor 760 includes the first gate
electrode 741 and the first oxide semiconductor layer 745 including
silicon oxide which overlaps with the first gate electrode 741 with
the gate insulating layer 743 interposed therebetween. The first
wiring 749 is a power supply line at a ground potential (a ground
power supply line). This power supply line at a ground potential
may be a power supply line to which a negative voltage VDL is
applied (a negative power supply line).
The second thin film transistor 761 includes the second gate
electrode 742 and the second oxide semiconductor layer 747
including silicon oxide which overlaps with the second gate
electrode 742 with the gate insulating layer 743 interposed
therebetween. The third wiring 751 is a power supply line to which
a positive voltage VDD is applied (a positive power supply
line).
As illustrated in FIG. 14A, the second wiring 750 which is
electrically connected to both the first oxide semiconductor layer
745 including silicon oxide and the second oxide semiconductor
layer 747 including silicon oxide is directly connected to the
second gate electrode 742 of the second thin film transistor 761
through the contact hole 744 formed in the gate insulating layer
743. Direct connection between the second wiring 750 and the second
gate electrode 742 can provide good contact and reduce the contact
resistance. In comparison with the case where the second gate
electrode 742 and the second wiring 750 are connected to each other
with a conductive film, for example, a transparent conductive film,
interposed therebetween, reduction in the number of contact holes
and reduction in an area occupied by the driver circuit in
accordance with the reduction in the number of contact holes can be
achieved.
A top view of the inverter circuit of the driver circuit is
illustrated in FIG. 14B. A cross section taken along dotted line
Y1-Y2 in FIG. 14B corresponds to FIG. 14A.
As in this embodiment, gallium is not used in the oxide
semiconductor layer, which leads to no use of gallium which is a
high cost material.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 6
This embodiment describes an example of a top-gate type thin film
transistor 330 with reference to FIGS. 15A and 15B. FIG. 15B
illustrates an example of a top view of a thin film transistor, a
cross-sectional view along dotted line P1-P2 of which corresponds
to FIG. 15A.
In FIG. 15A, over a substrate 300, a conductive film and an
oxynitride film are stacked, and etching is performed, so that a
first wiring 309 and a second wiring 310 are formed and oxynitride
layers 304a and 304b are formed thereover. The first wiring 309 and
the second wiring 310 function as source and drain electrodes. The
oxynitride layers 304a and 304b function as source and drain
regions and are formed using an In--Ga--Zn--O--N-based
non-single-crystal film.
Next, an oxide semiconductor layer 305 including silicon oxide
(SiO.sub.x) is formed to cover an exposed region of the substrate
300 and the oxynitride layers 304a and 304b. In this embodiment, a
Zn--O-based oxide semiconductor including silicon oxide is used as
the oxide semiconductor layer including silicon oxide.
Next, a gate insulating layer 303 covering the second oxide
semiconductor layer 305, the first wiring 309, and the second
wiring 310 is formed.
Next, heat treatment is preferably performed at 200.degree. C. to
600.degree. C., typically, 300.degree. C. to 500.degree. C. Here,
heat treatment is performed in air in a furnace at 350.degree. C.
for 1 hour. Through this heat treatment, rearrangement at the
atomic level occurs in the oxide semiconductor layer 305 including
silicon oxide. Because strain which inhibits carrier movement is
released by the heat treatment, the heat treatment (the heat
treatment may annealing with light) is important.
Next, a gate electrode 301 is provided in a position which is over
the gate insulating layer 303 and overlaps with a region where the
oxide semiconductor layer 305 including silicon oxide is in contact
with the substrate 300.
Through the above process, the top-gate type thin film transistor
330 can be manufactured.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 7
This embodiment describes an example of a top-gate type thin film
transistor 630 with reference to FIGS. 16A and 16B. FIG. 16B
illustrates an example of a top view of a thin film transistor, a
cross-sectional view along dotted line R1-R2 of which corresponds
to FIG. 16A.
In FIG. 16A, an oxide semiconductor layer 605 including silicon
oxide is formed over a substrate 600. In this embodiment, the oxide
semiconductor layer 605 is formed using a Zn--O-based oxide
semiconductor including silicon oxide.
Next, source and drain regions 606a and 606b are formed over the
oxide semiconductor layer 605. In this embodiment, the source and
drain regions 606a and 606b are formed using a Ga--Zn--O-based
non-single-crystal film. Alternatively, the source and drain
regions 606a and 606b may be formed using a Ga--Zn--O-based
non-single-crystal film including nitrogen, i.e., a
Ga--Zn--O--N-based non-single-crystal film (also called a GZON
film).
Next, a first wiring 609 and a second wiring 610 are formed over
the source and drain regions 606a and 606b. Note that the first and
second wirings 609 and 610 function as source and drain
electrodes.
Then, a gate insulating layer 603 is formed over the first and
second wirings 609 and 610.
Next, a gate electrode 601 is provided in a position which is over
the gate insulating layer 603 and overlaps with a region where the
oxide semiconductor layer 605 is in contact with the gate
insulating layer 603.
Next, heat treatment is preferably performed at 200.degree. C. to
600.degree. C., typically, 300.degree. C. to 500.degree. C. Here,
heat treatment is performed in air in a furnace at 350.degree. C.
for 1 hour. Through this heat treatment, rearrangement at the
atomic level occurs in the oxide semiconductor layer 605. Because
strain which inhibits carrier movement is released by the heat
treatment, the heat treatment (the heat treatment may be annealing
with light) is important.
Through the above process, the top-gate type thin film transistor
630 can be manufactured.
Embodiment 8
FIG. 31A is an example of a cross sectional view of a thin film
transistor in which an oxide semiconductor layer is sandwiched
between two gate electrodes provided over and below the oxide
semiconductor layer. This embodiment describes an example of a
manufacturing method by which thin film transistors used for a
pixel portion and a driver circuit are provided over a substrate
having an insulating surface.
First, a first gate electrode layer 11 is formed over a substrate
10 having an insulating surface. As the substrate 10 having an
insulating surface, any glass substrate used in the electronics
industry (also called an alkali-free glass substrate) such as an
aluminosilicate glass substrate, an aluminoborosilicate glass
substrate, or a barium borosilicate glass substrate, a plastic
substrate with heat resistance which can withstand a process
temperature in this manufacturing process, or the like can be used.
In the case where the substrate 10 is mother glass, the substrate
may have any of the following sizes: the first generation (320
mm.times.400 mm), the second generation (400 mm.times.500 mm), the
third generation (550 mm.times.650 mm), the fourth generation (680
mm.times.880 mm or 730 mm.times.920 mm), the fifth generation (1000
mm.times.1200 mm or 1100 mm.times.1250 mm), the sixth generation
(1500 mm.times.1800 mm), the seventh generation (1900 mm.times.2200
mm), the eighth generation (2160 mm.times.2460 mm), the ninth
generation (2400 mm.times.2800 mm or 2450 mm.times.3050 mm), the
tenth generation (2950 mm.times.3400 mm), and the like.
In addition, the material of the first gate electrode layer 11 can
be formed with a single layer or a stacked layer using a metal
material such as molybdenum, titanium, chromium, tantalum,
tungsten, aluminum, copper, neodymium, or scandium, or an alloy
material including any of the materials as a component. After the
conductive layer is formed over the entire surface of the substrate
10, a photolithography step is performed and a resist mask is
formed over the conductive layer. Then, unnecessary portions are
removed by etching to form wirings and electrodes (a gate wiring
including the first gate electrode layer 11, a capacitor wiring, a
terminal electrode, and the like). In this embodiment, a single
layer of tungsten having a thickness of 100 nm is used.
For example, in the case where the first gate electrode layer 11
has a stacked-layer structure, the following structures are
preferable: a two-layer structure of an aluminum layer and a
molybdenum layer stacked thereover, a two-layer structure of a
copper layer and a molybdenum layer stacked thereover, a two-layer
structure of a copper layer and a titanium nitride layer or a
tantalum nitride layer stacked thereover, and a two-layer structure
of a titanium nitride layer and a molybdenum layer. Alternatively,
a stack including a copper layer including Ca and a copper oxide
layer including Ca thereover, which serves as a barrier layer; or a
stack including a copper layer including Mg and a copper oxide
layer including Mg thereover, which serves as a barrier layer; can
be employed. Further alternatively, as a three-layer structure, a
stack of a tungsten layer or a tungsten nitride layer, a layer of
an alloy of aluminum and silicon or an alloy of aluminum and
titanium, and a titanium nitride layer or a titanium layer is
preferable.
Next, the resist mask is removed and then a gate insulating layer
13 covering the first gate electrode layer 11 is formed. The gate
insulating layer 13 is formed to a thickness of 50 nm to 400 nm by
a sputtering method, a PCVD method, or the like. The gate
insulating layer 13 is formed to have a single-layer structure or a
stacked-layer structure using an inorganic insulating film such as
a silicon oxide film, a silicon oxynitride film, a silicon nitride
oxide film, a silicon nitride film, or a tantalum oxide film. The
gate insulating layer 13 can be formed using a silicon oxide layer
by a CVD method using an organosilane gas. As an organosilane gas,
a silicon-containing compound such as tetraethoxysilane (TEOS)
(chemical formula: Si(OC.sub.2H.sub.5).sub.4), tetramethylsilane
(TMS) (chemical formula: Si(CH.sub.3).sub.4),
tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane
(OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (chemical
formula: SiH(OC.sub.2H.sub.5).sub.3), or trisdimethylaminosilane
(chemical formula: SiH(N(CH.sub.3).sub.2).sub.3) can be used.
In this embodiment, the gate insulating layer 13 having a thickness
of 100 nm is formed over the first gate electrode layer 11 as
follows: a monosilane gas (SiH.sub.4), nitrous oxide (N.sub.2O),
and a rare gas are introduced into a chamber of a high-density
plasma apparatus as source gases, and high density plasma is
generated under a pressure of 10 Pa to 30 Pa. In this embodiment,
the high-density plasma apparatus refers to an apparatus which can
realize a plasma density of 1.times.10.sup.11/cm.sup.3 or more. For
example, plasma is generated by applying a microwave power of 3 kW
to 6 kW so that the insulating film is formed. When the insulating
film is formed, the flow ratio of the monosilane gas (SiH.sub.4) to
the nitrous oxide (N.sub.2O) which are introduced into the chamber
is in the range of 1:10 to 1:200. In addition, as the rare gas
which is introduced into the chamber, helium, argon, krypton,
xenon, or the like can be used. In particular, argon, which is
inexpensive, is preferably used.
In addition, since the gate insulating layer 13 formed by using the
high-density plasma apparatus can have a uniform thickness, the
gate insulating layer 13 has excellent step coverage. Further, the
thickness of an insulating film formed by using the high-density
plasma apparatus can be controlled precisely.
The insulating film obtained by the high-density plasma apparatus
is greatly different from an insulating film formed by using a
conventional parallel plate PCVD apparatus. The insulating film
obtained by the high-density plasma apparatus has an etching rate
which is lower than that of the insulating film formed by using the
conventional parallel plate PCVD apparatus by 10% or more or 20% or
more in the case where the etching rates with the same etchant are
compared to each other. Thus, it can be said that the insulating
film obtained by using the high-density plasma apparatus is a dense
film.
Next, an oxide semiconductor film is formed over the gate
insulating layer 13. The thickness of the oxide semiconductor film
is at least 30 nm or more, preferably 60 nm or more and 150 nm or
less. In this embodiment, a first Zn--O-based non-single-crystal
film is formed as the oxide semiconductor film. In this embodiment,
the first Zn--O-based non-single-crystal film is formed under the
condition where a target is an oxide semiconductor target (ZnO)
including zinc (Zn) with a diameter of 8 inches, the distance
between the substrate and the target is set at 170 mm, the pressure
is set at 0.4 Pa, and the direct current (DC) power supply is set
at 0.5 kW in an argon atmosphere or an oxygen atmosphere. Note that
a pulse direct current (DC) power supply is preferable because dust
can be reduced and the film thickness can be uniform.
Note that in the case where a large-area glass substrate is used,
manufacturing in which one large target material is attached to one
large backing plate is difficult and costly. Therefore, the target
material is divided and bonded to a backing plate. The target is
formed by attaching the target material to a backing plate (a plate
for attaching a target material thereto) and vacuum packing. In
formation of the first Zn--O-based non-single-crystal film, in
order to obtain excellent electrical characteristics of a thin film
transistor, it is preferable that the backing plate including the
target material attached thereto is set in a sputtering apparatus
while being kept away from moisture and the like in the air as much
as possible. It is preferable that the target material is kept away
from moisture and the like in the air as much as possible not only
at the time of setting the target material to the sputtering
apparatus, but also during the period up to vacuum-packing, during
which manufacturing the target, bonding the target materials to the
backing plate, and the like are performed.
In the case where the Zn--O-based oxide semiconductor film is
formed by a sputtering method, the oxide semiconductor target
including Zn may include an insulating impurity such as silicon
oxide. Inclusion of the insulating impurity in the oxide
semiconductor facilitates amorphization of the oxide semiconductor
film to be formed. In addition, when the oxide semiconductor layer
is subjected to heat treatment in a later step, crystallization due
to the heat treatment can be suppressed.
Next, an oxide semiconductor film (in this embodiment, a second
Zn--O-based non-single-crystal film) which has lower resistance
than the first Zn--O-based non-single-crystal film is formed by a
sputtering method without exposure to the air. In this embodiment,
an oxynitride film including zinc is formed using an oxide
semiconductor target (ZnO) including zinc (Zn) in an atmosphere
including a nitrogen gas by a sputtering method. This oxynitride
film becomes an oxide semiconductor film which has lower resistance
than the first Zn--O-based non-single-crystal by heat treatment
performed later.
Next, a photolithography step is performed and a resist mask is
formed over the second Zn--O-based non-single-crystal film. Then,
the first and the second Zn--O-based non-single-crystal films are
etched. Note that etching here is not limited to wet etching and
dry etching may also be performed.
Next, the resist mask is removed and then a conductive film formed
from a metal material is formed over the first and the second
Zn--O-based non-single-crystal films by a sputtering method or a
vacuum evaporation method. As a material for the conductive film,
an element selected from Al, Cr, Ta, Ti, Mo, and W; an alloy
including any of the elements as a component; an alloy film
including a combination of any of the elements; and the like can be
given. Further, in the case where heat treatment is performed at
200 to 600.degree. C., the conductive film preferably has heat
resistance for such heat treatment. Since use of Al alone brings
disadvantages such as low heat resistance and a tendency to be
corroded, Al is used in combination with a conductive material
having heat resistance. As the conductive material having heat
resistance which is used in combination with Al, any of the
following materials may be used: an element selected from titanium
(Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr),
neodymium (Nd), and scandium (Sc), an alloy including any of the
elements as a component, an alloy film including a combination of
any of the elements, and a nitride including any of the elements as
a component.
Here, as the conductive film, a conductive film in which an Al film
and a Ti film are stacked is used. Alternatively, the conductive
film may be a single layer of a titanium film. Still alternatively,
the conductive film may have a three-layer structure including a Ti
film, an aluminum film including Nd (Al--Nd) which is stacked on
the Ti film, and a Ti film formed on these films. The conductive
film may have a single-layer structure of an aluminum film
including silicon.
Next, a photolithography step is performed and a resist mask is
formed over the conductive film. Then, unnecessary portions are
removed by etching, so that source and drain electrode layers 15a
and 15b are formed. Wet etching or dry etching is employed as an
etching method at this time. Here, dry etching is employed using a
mixed gas of SiCl.sub.4, Cl.sub.2, and BCl.sub.3 as a reactive gas
to etch the conductive film in which the Ti film and the Al film
are stacked. Thus, the source and drain electrode layers 15a and
15b are formed. In addition, in this etching, the second
Zn--O-based non-single-crystal film is selectively etched using the
same resist mask to form source and drain regions 14a and 14b, and
part of the first Zn--O-based non-single-crystal film is
exposed.
Through the above etching step using the resist mask, the exposed
first Zn--O-based non-single-crystal film is selectively etched. As
a result, an oxide semiconductor layer 16 including a region which
has a smaller thickness than a region overlapping with the source
electrode layer 15a or the drain electrode layer 15b is formed. The
source and drain electrode layers 15a and 15b, the source and drain
regions 14a and 14b, and the exposed first Zn--O-based
non-single-crystal film are etched in one step. Therefore, edge
portions of the source and drain electrode layers 15a and 15b and
the source and drain regions 14a and 14b are aligned and continuous
as illustrated in FIG. 1A. Note that the etching of the source and
drain electrode layers 15a and 15b, the source and drain regions
14a and 14b, the exposed first Zn--O-based non-single-crystal film
is not limited to the one-time etching. Alternatively, the etching
may be performed in a plurality of steps.
After the resist mask is removed, heat treatment at 200.degree. C.
to 600.degree. C., typically 300.degree. C. to 500.degree. C., is
preferably performed. In this case, heat treatment is performed in
a furnace at 350.degree. C. for 1 hour in a nitrogen atmosphere
including oxygen. Through this heat treatment, rearrangement at the
atomic level occurs in the first Zn--O-based non-single-crystal
film. Because strain which inhibits carrier movement is released by
the heat treatment, the heat treatment (the heat treatment may be
annealing with light) is important. In addition, resistance of the
second Zn--O-based non-single-crystal film is lowered and the
source and drain regions 14a and 14b having low resistance are
formed. There is no particular limitation on when to perform the
heat treatment as long as it is performed after the formation of
the second Zn--O-based non-single-crystal film.
Next, a resin layer 17 is formed to a thickness of 0.5 .mu.m to 3
.mu.m, which covers the source and drain electrode layers 15a and
15b and the oxide semiconductor layer 16 including the region
having a small thickness. As a photosensitive or non-photosensitive
organic material for the resin layer 17, polyimide, acrylic,
polyamide, polyimideamide, resist, benzocyclobutene, or a stack of
any of the materials is used. Here, photosensitive polyimide is
formed by a coating method for the purpose of reduction of the
number of steps. Exposure, development, and baking are performed
and the resin layer 17 formed form polyimide having a thickness of
1.5 .mu.m whose surface is flat is formed. The resin layer 17
functions as a first protective insulating layer which protects the
oxide semiconductor layer 16 including the region having a small
thickness and the source and drain regions 14a and 14b from plasma
damage in a later step of formation of a second protective
insulating layer. The resin layer 17 as the first protective
insulating layer, which covers and contact the exposed region
having a small thickness of the oxide semiconductor layer 16, also
has a function of blocking moisture, hydrogen, or the like from
entering the oxide semiconductor layer 16. The resin layer can be
formed without a pinhole and is good in terms of step coverage
because the resin layer can be formed to have a flat surface
regardless of unevenness of a surface over which the resin layer is
formed.
In addition, before formation of the resin layer 17, the exposed
region having a small thickness of the oxide semiconductor layer 16
may be subjected to oxygen radical treatment. By the oxygen radical
treatment, an exposed surface and its vicinity of the oxide
semiconductor layer can be modified into an oxygen-excess region.
Oxygen radicals may be produced in a plasma generation apparatus
with the use of a gas including oxygen, or in an ozone generation
apparatus. By exposing a thin film to the produced oxygen radicals
or oxygen, the surface of the film can be modified. The radical
treatment is not limited to one using oxygen radicals, and may be
performed using argon and oxygen radicals. The treatment using
argon and oxygen radicals is treatment in which an argon gas and an
oxygen gas are introduced to generate plasma, thereby modifying a
surface of a thin film.
Then, a second protective insulating layer 18 is formed to a
thickness of 50 nm to 400 nm by a PCVD method or a sputtering
method under a low power condition (or at a low substrate
temperature of lower than 200.degree. C., preferably a room
temperature to 100.degree. C.). Alternatively, the second
protective insulating layer 18 may be formed under a low power
condition using a high-density plasma apparatus. The second
protective insulating layer 18 obtained by a high-density plasma
apparatus can be denser than that obtained by using a PCVD method.
The second protective insulating layer 18, which is formed using a
silicon nitride film, a silicon oxynitride film, or a silicon
nitride oxygen film, blocks moisture, hydrogen ions, OH.sup.-, or
the like. Here, a silicon nitride film having a thickness of 200 nm
is formed by a PCVD method under the following conditions: the flow
rate of a silane gas is 35 sccm, the flow rate of ammonia
(NH.sub.3) is 300 sccm, and the flow rate of a hydrogen gas is 800
sccm; the pressure is 60 Pa, the RF electric power is 300 W; and
the power frequency is 13.56 MHz. These films have a function of
blocking moisture, hydrogen, OH.sup.-, or the like. When a second
gate electrode is selectively etched so as to have a top surface
having a desired shape, the second protective insulating film can
function as an etching stopper. In addition, in this case, the
first protective insulating film and the second protective
insulating film function as a second gate insulating layer.
Further, in the above structure, the region having a small
thickness of the oxide semiconductor layer is a channel formation
region overlapping with the first gate electrode and the second
gate electrode. In the region with a small thickness of the oxide
semiconductor layer, a region on the second gate electrode side is
referred to as a back channel. When film formation using plasma
including moisture, hydrogen, OH.sup.-, or the like is performed to
form a film in contact with the back channel, electric charge is
accumulated and negative charge or OH.sup.- of the plasma enters an
oxygen-deficiency-type defect portion in a buffer layer. As a
result, there is a possibility that an NI junction, which is
intended to be formed, is not formed. A lack of oxygen in the oxide
semiconductor layer increases Zn which is easy to receive negative
charges in the oxide semiconductor layer. When negative charge of
the plasma enters the oxygen-deficiency-type defect portion, the
buffer layer (an N.sup.+ region) is changed to an N-type region and
further, changed to an N.sup.- region or an I-type region. As a
result, an NI junction provided at an interface of the buffer layer
disappears. This possibly causes disappearance of a depletion layer
and an unstable value of Vg-Id characteristics of a thin film
transistor.
After a conductive layer is formed, a photolithography step is
performed and a resist mask is formed over the conductive layer.
Then, unnecessary portions are removed by etching and wirings and
electrodes (wirings including a second gate electrode layer 19 and
the like) are formed. When the second gate electrode layer 19 is
selectively etched so as to have a top surface having a desired
shape, the second protective insulating layer 18 can function as an
etching stopper.
As the conductive layer formed over the second protective
insulating layer 18, a metal material (an element selected from
aluminum (Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten
(W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium
(Sc), or an alloy including any of the elements as a component) can
be used. These films have a light-blocking property, and therefore
can block light to the oxide semiconductor layer.
In the cross section of FIG. 31A, the width of the second gate
electrode layer 19 is larger than that of the first gate electrode
layer 11 and larger than that of the oxide semiconductor layer. It
is effective that light is blocked by increasing the width of the
second gate electrode layer 19 than that of the oxide semiconductor
layer so that the second gate electrode layer 19 covers the top
surface of the oxide semiconductor layer. Since the region having a
small thickness of the oxide semiconductor layer 16 is not covered
with the source electrode or the drain electrode, there is a
possibility that the electric characteristics of the thin film
transistor are changed due to light irradiation. Since the
Zn--O-based non-single-crystal film formed by a sputtering method
is sensitive to light having a wavelength of 450 nm or less,
provision of the second gate electrode layer 19, which is a
light-blocking layer blocking light having a wavelength of 450 nm
or less, is useful.
Alternatively, the conductive layer formed over the second
protective insulating layer 18 can be formed using a
light-transmitting conductive material such as indium oxide
including tungsten oxide, indium zinc oxide including tungsten
oxide, indium oxide including titanium oxide, indium tin oxide
including titanium oxide, indium tin oxide (hereinafter referred to
as ITO), indium zinc oxide, or indium tin oxide to which silicon
oxide is added. In the case of using a light-transmitting
conductive material, when the same material as that used for a
pixel electrode is used, the gate electrode and the pixel electrode
can be formed using the same photomask. When the second gate
electrode and the pixel electrode are formed using the same
material, the number of steps can be reduced. In addition, in the
case where the second gate electrode is formed using a
light-transmitting conductive material, it is preferable that a
light-blocking layer for shielding the oxide semiconductor layer 16
including the region having a small thickness from light is
separately formed so as to overlap the region with a small
thickness of the oxide semiconductor layer 16. A material having a
light transmittance of at least less than 50%, preferably less than
20% at a wavelength of 400 nm to 450 nm is used for the
light-blocking layer. For example, a metal film of chromium or
titanium nitride or a black resin can be used as a material of the
light-blocking layer. In the case of using a black resin for
blocking light, as the light intensity is higher, the film of the
black resin needs to be thicker. Therefore, in the case where the
film of the black resin needs to be thin, a metal film which has a
high light-blocking property and can be subjected to a fine etching
process and can be thinned is preferably used.
Through the above process, a thin film transistor 20 illustrated in
FIG. 31A can be obtained.
A general photomask is used for the photolithography step in an
example described in the above. When a resist mask having a
plurality of thicknesses (typically, two kinds of thicknesses),
which is formed by a photolithography step using a multi-tone mask,
is used, the number of resist masks can be reduced and therefore
the process can be simplified and cost can be reduced.
In addition, when the second gate electrode layer 19 and the first
gate electrode layer 11 are electrically connected to each other in
order that the second gate electrode layer 19 and the first gate
electrode layer 11 have the same potential, before the second gate
electrode layer 19 is formed over the second protective insulating
layer 18, a photolithography step is performed, a resist mask is
formed over the second protective insulating layer 18, and
unnecessary portions are removed by etching, so that an opening
reaching the first gate electrode layer 11 is formed.
Note that in the case where the second gate electrode layer 19 and
the first gate electrode layer 11 have different potential, the
opening for electrical connection of the second gate electrode
layer 19 and the first gate electrode layer 11 is not required.
FIG. 31B is partly different from FIG. 31A. In FIG. 31B, the same
portions as those of FIG. 31A other than different portions are
denoted by the same reference numerals.
FIG. 31B illustrates an example in which the second gate electrode
layer 19 and the second protective insulating layer 18 are formed
in an order different from FIG. 31A.
As illustrated in FIG. 31B, the second gate electrode layer 19 of a
thin film transistor 21 is formed over and in contact with the
resin layer 17 that is the first protective insulating film and
provided between the resin layer 17 and the second protective
insulating layer 18. The second gate insulating layer of the thin
film transistor 20 in FIG. 31A is a stack of the resin layer 17 and
the second protective insulating layer 18, whereas the second gate
insulating layer of the thin film transistor 21 is the resin layer
17 alone. In the case where the second gate electrode layer 19 is
provided between the resin layer 17 and the second protective
insulating layer 18, the second gate electrode layer 19 as well as
the resin layer 17 has an effect of reducing plasma damage to the
oxide semiconductor layer 16.
In addition, FIG. 31B illustrates an example in which a base
insulating layer 12 is provided between the first gate electrode
layer 11 and the substrate 10. In the case where a silicon
oxynitride film, a silicon nitride oxide film, or a silicon nitride
film, or the like having a thickness of 50 nm to 200 nm is used as
the base insulating layer 12, the base insulating layer 12 can
block an impurity from the glass substrate, sodium for example,
from diffusing into and entering an oxide semiconductor to be
formed later over the base insulating layer 12. In addition, in the
case where the base insulating layer 12 is provided, the substrate
10 can be prevented from being etched in the etching step for
forming the first gate electrode layer 11.
Further, an example of the channel-etch type thin film transistor
which is one kind of an inverted-staggered thin film transistor is
described in the above structure; however, there is no particular
limitation on the structure of the thin film transistor. For
example, a bottom-contact thin film transistor may be employed. An
oxide semiconductor layer of a bottom-contact structure is formed
after source and drain electrodes are formed by selectively etching
a conductive film; therefore, the number of steps after formation
of the oxide semiconductor layer is small and the number of
exposure of the oxide semiconductor layer to plasma is also small
as compared to the case of a channel-etch TFT. As the number of
exposure to plasma is small, plasma damage to the oxide
semiconductor layer can be reduced.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 9
FIG. 32A is an example of a cross sectional view of a thin film
transistor in which an oxide semiconductor layer is sandwiched
between two gate electrodes provided over and below the oxide
semiconductor layer. This embodiment describes an example of a
manufacturing method by which thin film transistors used for a
pixel portion and a driver circuit are provided over a substrate
having an insulating surface.
The same steps as Embodiment 8 are employed from formation of a
first gate electrode layer 11 over a substrate 10 having an
insulating surface up to formation of a gate insulating layer 13
covering the first gate electrode layer 11. Therefore, detailed
description is omitted and the same portions as those of FIG. 31A
are denoted by the same reference numerals.
A conductive film is formed from a metal material over the gate
insulating layer 13 by a sputtering method or a vacuum evaporation
method. In this embodiment, a three-layer structure of a Ti film,
an aluminum film including Nd, and a Ti film formed by a sputtering
method is employed. As a material for the conductive film, an
element selected from Al, Cr, Ta, Ti, Mo, and W; an alloy including
any of the elements as a component; an alloy film including a
combination of any of the elements; and the like can be given.
Further, the conductive film may have a two-layer structure, and a
titanium film may be stacked over an aluminum film. Alternatively,
the conductive film may have a single-layer structure of an
aluminum film including silicon or a single-layer structure of a
titanium film.
Then, an oxide semiconductor film (a buffer layer) having low
resistance is formed by a sputtering method without exposure to the
air. There is no particular limitation on a material film of the
buffer layer as long as it has lower resistance than an oxide
semiconductor layer 26 formed later. As the buffer layer, an
oxynitride film including zinc is formed over the conductive film.
The oxynitride film including zinc is obtained by using an oxide
semiconductor target (ZnO) including zinc (Zn) in an atmosphere
including a nitrogen gas by a sputtering method. In this
embodiment, the buffer layer is formed to a thickness of 10 nm
using an oxide semiconductor target (ZnO) under the following
conditions: the flow rate of Ar is 72 sccm, the flow rate of oxygen
is 3 sccm, the electric power is 3.2 kw, and the pressure is 0.16
Pa. Note that in order to reduce plasma damage to the buffer layer,
the electric power may be reduced to 1 kw at the formation.
Examples of a sputtering method include an RF sputtering method in
which a high-frequency power source is used as a sputtering power
source, a DC sputtering method, and a pulsed DC sputtering method
in which a bias is applied in a pulsed manner. An RF sputtering
method is mainly used in the case where an insulating film is
formed, and a DC sputtering method is mainly used in the case where
a metal film is formed.
In addition, there is also a multi-source sputtering apparatus in
which a plurality of targets of different materials can be set.
With the multi-source sputtering apparatus, films of different
materials can be formed to be stacked in the same chamber, or a
film of plural kinds of materials can be formed by electric
discharge at the same time in the same chamber.
In addition, there are a sputtering apparatus provided with a
magnet system inside the chamber and used for a magnetron
sputtering, and a sputtering apparatus used for an ECR sputtering
in which plasma generated with the use of microwaves is used
without using glow discharge.
Furthermore, as a deposition method by sputtering, there are also a
reactive sputtering method in which a target substance and a
sputtering gas component are chemically reacted with each other
during deposition to form a thin compound film thereof, and a bias
sputtering in which a voltage is also applied to a substrate during
deposition.
The target is formed by attaching a target material to a backing
plate (a plate for attaching a target material thereto). As for the
attachment of the target material to the backing plate, the target
material may be divided and attached to one backing plate. A case
where four target materials are attached to one backing plate is
referred to as four divisions. Further, a case where nine target
materials are attached to one backing plate is referred to as nine
divisions. There is no particular limitation of the number of
divisions of target materials. When the target material is divided,
warpage of the target material can be relaxed in the attachment of
the target material to the backing plate. In particular, when the
thin film is formed over a large substrate, such divided target
materials can be suitably used for a target which is upsized in
accordance with the size of the large substrate. Needless to say,
one target material may be attached to one backing plate.
Next, a photolithography step is performed to form a resist mask
over the buffer layer, and unnecessary portions are removed by
etching to form source and drain electrode layers 25a and 25b. The
buffer layer whose top surface has the same shape as the source and
drain electrode layers 25a and 25b remains over the source and
drain electrode layers 25a and 25b. After that, the resist mask is
removed.
Next, an oxide semiconductor film having a thickness of 5 nm to 200
nm is formed. In this embodiment, the oxide semiconductor film is
formed to a thickness of 50 nm using an oxide semiconductor target
(ZnO) including silicon oxide (SiO.sub.x) and zinc (Zn) under the
following formation conditions: the flow rate of Ar is 50 sccm, the
flow rate of oxygen is 20 sccm, the electric power is 1 kw, and the
pressure is 0.22 Pa.
In addition, before the oxide semiconductor film is formed, plasma
treatment for removing dust attached to surfaces of the source and
drain electrode layers 25a and 25b and the gate insulating layer is
preferably performed. For example, the plasma treatment is
performed on the exposed source and drain electrode layers 25a and
25b and the exposed gate insulating layer by performing reverse
sputtering in which plasma is generated by an RF power supply by
introduction of an argon gas.
Next, a photolithography step is performed to form a resist mask
over the oxide semiconductor film, and unnecessary portions are
removed by etching to form the oxide semiconductor layer 26. In
addition, the buffer layer is selectively etched using the same
resist mask and source and drain regions 24a and 24b are
formed.
Then, after the resist mask is removed, heat treatment at
200.degree. C. to 600.degree. C., typically 300.degree. C. to
500.degree. C., is preferably performed. Here, heat treatment is
performed in a furnace at 350.degree. C. for 1 hour in a nitrogen
atmosphere including oxygen. Through this heat treatment,
rearrangement at the atomic level occurs in the Zn--O-based
non-single-crystal film. Because strain which inhibits carrier
movement is released by the heat treatment, the heat treatment (the
heat treatment may be annealing with light) is important.
A resin layer 17 covering the source and drain electrode layers 25a
and 25b and the oxide semiconductor layer 26 is formed to a
thickness in the range of 5 .mu.m to 3 .mu.m. As a photosensitive
or non-photosensitive organic material for the resin layer 17,
polyimide, acrylic, polyamide, polyimideamide, resist,
benzocyclobutene, or a stack of any of these materials is used.
Note that steps after formation of the resin layer 17 are the same
as those of Embodiment 8 and therefore, are described briefly
here.
Then, a second protective insulating layer 18 is formed to a
thickness of 50 nm to 400 nm by a PCVD method or a sputtering
method under a low power condition (or at a low substrate
temperature of lower than 200.degree. C., preferably a room
temperature to 100.degree. C.). Alternatively, the second
protective insulating layer 18 may be formed under a low power
condition using a high-density plasma apparatus.
After a conductive layer is formed, a photolithography step is
performed and a resist mask is formed over the conductive layer.
Then, unnecessary portions are removed by etching and wirings and
electrodes (wirings including a second gate electrode layer 19 and
the like) are formed.
Through the above process, a thin film transistor 22 illustrated in
FIG. 32A can be obtained.
FIG. 32B is partly different from FIG. 32A. In FIG. 32B, the same
portions as those of FIG. 32A other than different portions are
denoted by the same reference numerals.
FIG. 32B illustrates an example in which the second gate electrode
layer 19 and the second protective insulating layer 18 are formed
in an order different from FIG. 32A.
As illustrated in FIG. 32B, the second gate electrode layer 19 of a
thin film transistor 23 is formed over and in contact with the
resin layer 17 that is the first protective insulating film and
provided between the resin layer 17 and the second protective
insulating layer 18. In the case where the second gate electrode
layer 19 is provided between the resin layer 17 and the second
protective insulating layer 18, the second gate electrode layer 19
as well as the resin layer 17 has an effect of reducing plasma
damage to the oxide semiconductor layer 26.
FIG. 32C is partly different from FIG. 32A. In FIG. 32C, the same
portions as those of FIG. 32A other than different portions are
denoted by the same reference numerals.
FIG. 32C illustrates an example which differs from FIG. 32A in
positional relation between source and drain regions 27a and 27b
and source and drain electrode layers 28a and 28b. The source and
drain regions 27a and 27b are provided below the source and drain
electrode layers 28a and 28b. The source and drain electrode layers
28a and 28b have an effect of reducing plasma damage to the source
and drain regions 27a and 27b.
In other words, as a blocking layer for reducing plasma damage to
the source and drain regions 27a and 27b, three layers (the source
and drain electrode layers 28a and 28b, the resin layer 17, and the
second gate electrode layer 19) are formed over the source and
drain regions 27a and 27b; therefore, plasma damage to the source
and drain regions 27a and 27b is further reduced.
As for a thin film transistor 29 illustrated in FIG. 32C, an oxide
semiconductor film having low resistance is formed over and in
contact with the gate insulating layer 13 and a conductive film is
formed thereover. After that, the oxide semiconductor film having
low resistivity is etched using the same resist mask as that used
for selectively etching the conductive film. Therefore, top
surfaces of the source and drain regions 27a and 27b which are
formed by etching the oxide semiconductor film having low
resistance have approximately the same shape as top surfaces of the
source and drain electrode layers 28a and 28b which are formed over
the source and drain regions 27a and 27b. The top surfaces and side
surfaces of the source and drain electrode layers 28a and 28b are
formed in contact with the oxide semiconductor layer 26.
FIG. 32D is partly different from FIG. 32C. In FIG. 32D, the same
portions as those of FIG. 32C other than different portions are
denoted by the same reference numerals.
FIG. 32D illustrates an example in which the second gate electrode
layer 19 and the second protective insulating layer 18 are formed
in an order different from FIG. 32C.
As illustrated in FIG. 32D, the second gate electrode layer 19 of a
thin film transistor 30 is formed over and in contact with the
resin layer 17 that is the first protective insulating film and
provided between the resin layer 17 and the second protective
insulating layer 18. When the second gate electrode layer 19 is
provided between the resin layer 17 and the second protective
insulating layer 18, the second gate electrode layer 19 as well as
the resin layer 17 has an effect of reducing plasma damage to the
oxide semiconductor layer 26.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 10
FIG. 33A is an example of a cross sectional view of a thin film
transistor in which an oxide semiconductor layer is sandwiched
between two gate electrode provided over and below the oxide
semiconductor layer. This embodiment describes an example of a
manufacturing method by which thin film transistors used for a
pixel portion and a driver circuit are provided over a substrate
having an insulating surface.
Note that steps in which a first gate electrode 11 is formed over a
substrate having an insulating surface, a gate insulating layer 13
covering the first gate electrode layer 11 is formed, and an oxide
semiconductor film is formed are the same as those of Embodiment 8.
Therefore, detailed description is omitted and the same portions as
those of FIG. 31A are denoted by the same reference numerals.
In this embodiment, an oxide semiconductor film over the gate
insulating layer 13 is formed using a Zn--O-based oxide
semiconductor target including silicon oxide at 5 wt % to 50 wt %
inclusive, preferably 10 wt % to 30 wt % inclusive, so that the
Zn--O-based oxide semiconductor film includes silicon oxide
(SiO.sub.x (X>0)) which interrupts crystallization.
Then, a channel protective film is formed over the Zn--O-based
oxide semiconductor film by a sputtering method without exposure to
the air. As a material of the channel protective film, an inorganic
material (a silicon oxide film, a silicon nitride film, a silicon
oxynitride film, a silicon nitride oxide film, or the like) can be
used.
Note that a silicon oxynitride film refers to a film that includes
more oxygen than nitrogen in the case where measurements are
performed using Rutherford backscattering spectrometry (RBS) and
hydrogen forward scattering (HFS). In addition, a silicon nitride
oxide film refers to a film that includes more nitrogen than oxygen
in the case where measurements are performed using RBS and HFS.
Next, a photolithography step is performed and a resist mask is
formed over the channel protective film. Then, unnecessary portions
are removed by etching and a channel protective layer 43 is formed.
Note that the width of the first gate electrode layer 11 is larger
than the width of the channel protective layer 43 (the width in the
channel length direction).
As a material of the channel protective layer 43, not only an
inorganic insulating material but also an amorphous semiconductor
film or a compound thereof obtained by a sputtering method,
typically an amorphous silicon film can be used. A compound of an
amorphous silicon film used for the channel protective layer refers
to a p-type amorphous silicon film formed by a sputtering method,
which includes a p-type impurity element such as boron, or an
n-type amorphous silicon film formed by a sputtering method, which
includes an n-type impurity element such as phosphorus. If a p-type
amorphous silicon film is used as the channel protective layer 43,
leakage current in an off state can be reduced and carriers
(electrons) generated in the oxide semiconductor layer provided in
contact with the p-type amorphous silicon film can be cancelled. In
the case where an amorphous silicon film is used as the channel
protective layer 43, the amorphous silicon film has a blocking
function against moisture, hydrogen ions, OH.sup.-, or the like. In
addition, the amorphous silicon film functions as a light-blocking
layer for blocking incidence of light to the oxide
semiconductor.
In this embodiment, an amorphous silicon film including boron
obtained by a sputtering method using a target including boron is
used as the channel protective layer 43. The amorphous silicon film
including boron is formed in a low power condition or at a
substrate temperature of lower than 200.degree. C. Since the
channel protective layer 43 is formed in contact with the
Zn--O-based non-single-crystal film, damage to the Zn--O-based
non-single-crystal film at the time of forming and etching the
channel protective layer 43 is preferably reduced as much as
possible.
Next, an oxide semiconductor film (an In--Ga--Zn--O--N-based
non-single-crystal film in this embodiment) having lower resistance
than the Zn--O-based non-single-crystal film is formed over the
Zn--O-based non-single-crystal film and the protective layer 43 by
a sputtering method. In this embodiment, sputtering is performed in
an atmosphere including a nitrogen gas, with use of an oxide
semiconductor target including In (indium), Ga (gallium), and Zn
(zinc) (In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:1), so that an
oxynitride film including indium, to gallium, and zinc is formed.
The oxynitride film becomes the oxide semiconductor film having low
resistance by heat treatment performed later.
Next, a photolithography step is performed and a resist mask is
formed over the In--Ga--Zn--O--N-based non-single-crystal film.
Then, the Zn--O-based non-single-crystal film and the
In--Ga--Zn--O--N-based non-single-crystal film are etched. After
the etching, a side surface of an oxide semiconductor layer 44
formed from the Zn--O-based non-single-crystal film is exposed.
Note that etching here is not limited to wet etching and dry
etching may also be performed.
Then, after the resist mask is removed, a conductive film formed
from a metal material is formed over the In--Ga--Zn--O--N-based
non-single-crystal film by a sputtering method or a vacuum
evaporation method. As a material for the conductive film, an
element selected from Al, Cr, Ta, Ti, Mo, and W; an alloy including
any of the elements as a component; an alloy film including a
combination of any of the elements; and the like can be given.
Further, for heat treatment at 200.degree. C. to 600.degree. C.,
the conductive film preferably has heat resistance for such heat
treatment.
A photolithography step is performed and a resist mask is formed
over the conductive film. Unnecessary portions are removed by
etching, and source and drain electrode layers 36a and 36b are
formed. In this etching, the channel protective layer 43 functions
as an etching stopper of the oxide semiconductor layer 44.
Therefore, the oxide semiconductor layer 44 is not etched. In
addition, in this etching, the In--Ga--Zn--O--N-based
non-single-crystal film is selectively etched using the same mask
and source and drain regions 35a and 35b are formed.
Because of the structure in which the channel protective layer 43
is provided over a channel formation region of the oxide
semiconductor layer 44, damage to the channel formation region of
the oxide semiconductor layer 44 (for example, reduction in
thickness due to plasma or an etchant in etching, or oxidation) in
the manufacturing process can be prevented. Therefore, reliability
of a thin film transistor 31 can be improved.
Then, after the resist mask is removed, heat treatment at
200.degree. C. to 600.degree. C., typically 300.degree. C. to
500.degree. C., is preferably performed. In this embodiment, heat
treatment is performed in a furnace at 350.degree. C. for 1 hour in
a nitrogen atmosphere or a nitrogen atmosphere including
oxygen.
Next, a resin layer 17 is formed to a thickness of 0.5 .mu.m to 3
.mu.m, which covers the source and drain electrode layers 36a and
36b and the channel protective layer 43. As a photosensitive or non
photosensitive organic material used for the resin layer 17,
polyimide, acrylic, polyamide, polyimideamide, resist,
benzocyclobutene, or a stack of any of these materials is used.
Note that steps after formation of the resin layer 17 are the same
as those of Embodiment 8 and therefore, are briefly described
here.
Then, a second protective insulating layer 18 is formed to a
thickness of 50 nm to 400 nm by a PCVD method or a sputtering
method under a low power condition (or at a low substrate
temperature of lower than 200.degree. C., preferably a room
temperature to 100.degree. C.). Alternatively, the second
protective insulating layer 18 may be formed under a low power
condition using a high-density plasma apparatus.
After a conductive layer is formed, a photolithography step is
performed and a resist mask is formed over the conductive layer.
Then, unnecessary portions are removed by etching and wirings and
electrodes (wirings including a second gate electrode layer 19 and
the like) are formed.
Through the above process, the thin film transistor 31 illustrated
in FIG. 33A can be obtained. Note that in the thin film transistor
31, a stack of the channel protective layer 43, the resin layer 17,
and the second protective insulating layer 18 functions as the
second gate insulating layer.
By making the width of the first gate electrode layer 11 larger
than that of the second gate electrode layer 19, a gate voltage can
be applied from the second gate electrode layer 19 to the whole
oxide semiconductor layer 44. In addition, in the case where
parasitic capacitance does not cause a problem, the second gate
electrode layer may cover a plurality of thin film transistors in
the driver circuit and the area of the second gate electrode layer
may be approximately the same as or larger than that of the driver
circuit.
In the case where the parasitic capacitance causes a problem, in
the structure of FIG. 33A, it is preferable that the width of the
first gate electrode layer 11 is set to be smaller than that of the
second gate electrode layer 19 so that an area of the first gate
electrode layer 11 which overlaps with the source and drain
electrode layers is reduced, whereby the parasitic capacitance is
reduced. Further, the width of the first gate electrode layer 11
may be set to be larger than the width of the channel protective
layer 43 and the width of the second gate electrode layer 19 may be
set to be smaller than the width of the channel protective layer 43
so that the second gate electrode layer 19 does not overlap with
the source drain electrode layers, whereby more parasitic
capacitance may be reduced.
FIG. 33B is partly different from FIG. 33A. In FIG. 33B, the same
portions as those of FIG. 33A other than different portions are
denoted by the same reference numerals.
FIG. 33B illustrates an example in which the second gate electrode
layer 19 and the second protective insulating layer 18 are formed
in an order different from FIG. 33A.
As illustrated in FIG. 33B, the second gate electrode layer 19 of a
thin film transistor 32 is formed over and in contact with the
resin layer 17 that is the first protective insulating film and
provided between the resin layer 17 and the second protective
insulating layer 18. In the case where the second gate electrode
layer 19 is provided between the resin layer 17 and the second
protective insulating layer 18, the second gate electrode layer 19
as well as the resin layer 17 has an effect of reducing plasma
damage to the oxide semiconductor layer 44.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 11
FIG. 34A is an example of a cross-sectional view of a thin film
transistor in which an oxide semiconductor layer is sandwiched
between two gate electrode layers provided over and below the oxide
semiconductor layer. This embodiment describes an example of a thin
film transistor used for a pixel portion and a driver circuit which
are provided over a substrate having an insulating surface.
Note that this embodiment is the same as Embodiment 8 except that
an amorphous silicon film is provided in contact with an oxide
semiconductor layer 16. Therefore, detailed description is omitted
here and the same portions as those of FIG. 31A are denoted by the
same reference numerals. Steps are the same as those of Embodiment
8 before forming a region with a small thickness in the oxide
semiconductor layer 16 by partly etching the oxide semiconductor
layer 16 using source and drain electrode layers 15a and 15b as a
mask.
According to Embodiment 8, the oxide semiconductor layer 16
including a region with a smaller thickness than a region
overlapping with the source and drain electrode layers 15a and 15b
is formed.
Then, after removing the resist mask, an amorphous semiconductor
film or a compound thereof obtained by a sputtering method,
typically an amorphous silicon film is formed. Note that a compound
of an amorphous silicon film refers to a p-type amorphous silicon
film formed by a sputtering method which includes a p-type impurity
element such as boron, or an n-type amorphous silicon film formed
by a sputtering method which includes an n-type impurity element
such as phosphorus.
In order to reduce damage to the oxide semiconductor layer 16 as
much as possible, the film is formed under a low power condition or
a condition where a substrate temperature is lower than 200.degree.
C. In this embodiment, the amorphous silicon film is formed with a
substrate temperature set at room temperature and electric power
set at 1 kw.
In addition, before formation of the amorphous silicon film, the
exposed region having a small thickness of the oxide semiconductor
layer 16 may be subjected to oxygen radical treatment. By the
oxygen radical treatment, an exposed surface and its vicinity of
the oxide semiconductor layer can be modified into an oxygen-excess
region. If the amorphous silicon film is formed on the
oxygen-excess region formed by the oxygen radical treatment, a thin
film of SiO.sub.x (X>0) is formed at an interface, which can
reduce off current.
Oxygen radicals may be produced in a plasma generation apparatus
with the use of a gas including oxygen, or in an ozone generation
apparatus. By exposing a thin film to the produced oxygen radicals
or oxygen, the surface of the film can be modified. The radical
treatment is not limited to one using oxygen radicals, and may be
performed using argon and oxygen radicals. The treatment using
argon and oxygen radicals is treatment in which an argon gas and an
oxygen gas are introduced to generate plasma, thereby modifying a
surface of a thin film.
Next, a photolithography step is performed to form a resist mask
over the amorphous silicon film. Then, unnecessary portions are
removed by etching and a channel protective layer 41 is formed.
Note that an example in which the amorphous silicon film is
selectively etched is described in this embodiment without
particular limitations. A photolithography step here may be omitted
in order to reduce the number of photomasks and steps. The channel
protective layer 41 can be used as an interlayer film which blocks
moisture, hydrogen ions, OH.sup.-, or the like. In addition, the
channel protective layer 41 formed of the amorphous silicon film
functions as a light-blocking layer for blocking incidence of light
to the oxide semiconductor layer.
Then, the resin layer 17 is formed with a thickness in the range of
0.5 .mu.m to 3 .mu.m to cover the source and drain electrode layers
15a and 15b and the channel protective layer 41. As a
photosensitive or non photosensitive organic material for the resin
layer 17, polyimide, acrylic, polyamide, polyimideamide, resist,
benzocyclobutene, or a stack of any of these materials is used.
Note that steps after formation of the resin layer 17 are the same
as those of Embodiment 8 and therefore, are briefly described
here.
A second protective insulating layer 18 is formed to a thickness of
50 nm to 400 nm by a PCVD method or a sputtering method under a low
power condition (or at a low substrate temperature of lower than
200.degree. C., preferably a room temperature to 100.degree. C.).
Alternatively, the second protective insulating layer 18 may be
formed under a low power condition using a high-density plasma
apparatus.
After a conductive layer is formed, a photolithography step is
performed to form a resist mask over the conductive layer and
unnecessary portions are removed by etching so that wirings and
electrodes (wirings including a second gate electrode layer 19 and
the like) are formed.
Through the above process, a thin film transistor 31 illustrated in
FIG. 34A can be obtained.
In addition, the channel protective layer 41 formed of the
amorphous silicon film also functions as a light-blocking layer for
blocking incidence of light to the oxide semiconductor layer. In
this embodiment, an example is shown in which an amorphous silicon
film is used as the channel protective layer 41. If a p-type
amorphous silicon film is used as the channel protective layer 41,
leakage current in an off state can be reduced and carriers
(electrons) generated in the oxide semiconductor layer provided in
contact with the p-type amorphous silicon film can be
cancelled.
FIG. 34B is partly different from FIG. 34A. In FIG. 34B, the same
portions as those of FIG. 34A other than different portions are
denoted by the same reference numerals.
FIG. 34B illustrates an example in which the second gate electrode
layer 19 and the second protective insulating layer 18 are formed
in an order different from FIG. 34A.
As illustrated in FIG. 34B, the second gate electrode layer 19 of a
thin film transistor 32 is formed over and in contact with the
resin layer 17 that is a first protective insulating film and
provided between the resin layer 17 and the second protective
insulating layer 18. In the case where the second gate electrode
layer 19 is provided between the resin layer 17 and the second
protective insulating layer 18, the second gate electrode layer 19
as well as the channel protective layer 41 and the resin layer 17
has an effect of reducing plasma damage to the oxide semiconductor
layer 16.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 12
FIG. 35A is an example of a cross-sectional view of a thin film
transistor in which an oxide semiconductor layer is sandwiched
between two gate electrode layers provided over and below the oxide
semiconductor layer. This embodiment describes an example of a thin
film transistor used for a pixel portion and a driver circuit which
are provided over a substrate having an insulating surface.
Note that this embodiment is the same as Embodiment 9 except that
an amorphous silicon film is provided in contact with an oxide
semiconductor layer 26. Therefore, detailed description is omitted
here and the same portions as those of FIG. 32A are denoted by the
same reference numerals. Steps are the same as those of Embodiment
9 up to formation of the oxide semiconductor layer which is partly
in contact with a gate insulating layer 13.
After forming the oxide semiconductor film according to Embodiment
9, an amorphous semiconductor film or a compound thereof obtained
by a sputtering method, typically an amorphous silicon film is
formed without exposure to the air. Note that a compound of an
amorphous silicon film refers to a p-type amorphous silicon film
formed by a sputtering method which includes a p-type impurity
element such as boron, or an n-type amorphous silicon film formed
by a sputtering method which includes an n-type impurity element
such as phosphorus.
In order to reduce damage to the oxide semiconductor layer 26 as
much as possible, the film is formed under a low power condition or
a condition where a substrate temperature is lower than 200.degree.
C. In this embodiment, the amorphous silicon film including boron
is formed with a substrate temperature set at room temperature and
electric power set at 1 kw.
In addition, before formation of the amorphous silicon film
including boron, an exposed region of the oxide semiconductor film
may be subjected to oxygen radical treatment. By the oxygen radical
treatment, a surface and its vicinity of the oxide semiconductor
film can be modified into an oxygen-excess region. If the amorphous
silicon film is formed on the oxygen-excess region formed by the
oxygen radical treatment, a thin film of SiO.sub.x (X>0) is
formed at an interface, which can reduce off current.
Oxygen radicals may be produced in a plasma generation apparatus
with the use of a gas including oxygen, or in an ozone generation
apparatus. By exposing a thin film to the produced oxygen radicals
or oxygen, the surface of the film can be modified. The radical
treatment is not limited to one using oxygen radicals, and may be
performed using argon and oxygen radicals. The treatment using
argon and oxygen radicals is treatment in which an argon gas and an
oxygen gas are introduced to generate plasma, thereby modifying a
surface of a thin film.
Next, a photolithography step is performed to form a resist mask
over the amorphous silicon film including boron. Then, unnecessary
portions are removed by etching and a channel protective layer 42
is formed. The channel protective layer 42 can be used as an
interlayer film which blocks moisture, hydrogen ions, OH.sup.-, or
the like. In addition, the channel protective layer 42 formed from
the amorphous silicon film functions as a light-blocking layer for
blocking incidence of light to the oxide semiconductor layer. In
addition, unnecessary portions of the oxide semiconductor film are
removed using the same resist mask and the oxide semiconductor
layer 26 is formed. Further, the buffer layer is selectively etched
using the same mask and source and drain regions 24a and 24b are
formed.
After the resist mask is removed, heat treatment at 200.degree. C.
to 600.degree. C., typically 300.degree. C. to 500.degree. C., is
preferably performed. In this embodiment, heat treatment is
performed in a furnace at 350.degree. C. for 1 hour in a nitrogen
atmosphere including oxygen.
Then, a resin layer 17 is formed with a thickness in the range of
0.5 .mu.m to 3 .mu.m to cover source and drain electrode layers 25a
and 25b and the oxide semiconductor layer 26. As a photosensitive
or non photosensitive organic material for the resin layer 17,
polyimide, acrylic, polyamide, polyimideamide, resist,
benzocyclobutene, or a stack of any of these materials is used.
Note that steps after formation of the resin layer 17 are the same
as those of Embodiment 9 and therefore, are briefly described
here.
A second protective insulating layer 18 is formed to a thickness of
50 nm to 400 nm by a PCVD method or a sputtering method under a low
power condition (or at a low substrate temperature of 200.degree.
C., preferably a room temperature to 100.degree. C.).
Alternatively, the second protective insulating layer 18 may be
formed under a low power condition using a high-density plasma
apparatus.
After a conductive layer is formed, a photolithography step is
performed to form a resist mask over the conductive layer and
unnecessary portions are removed by etching so that wirings and
electrodes (wirings including a second gate electrode layer 19 and
the like) are formed.
Through the above process, a thin film transistor 33 illustrated in
FIG. 35A can be obtained.
FIG. 35B is partly different from FIG. 35A. In FIG. 35B, the same
portions as those of FIG. 35A other than different portions are
denoted by the same reference numerals.
FIG. 35B illustrates an example in which the second gate electrode
layer 19 and the second protective insulating layer 18 are formed
in an order different from FIG. 35A.
As illustrated in FIG. 35B, the second gate electrode layer 19 of a
thin film transistor 34 is formed over and in contact with the
resin layer 17 that is a first protective insulating film and
provided between the resin layer 17 and the second protective
insulating layer 18. In the case where the second gate electrode
layer 19 is provided between the resin layer 17 and the second
protective insulating layer 18, the second gate electrode layer 19
as well as the channel protective layer 42 and the resin layer 17
has an effect of reducing plasma damage to the oxide semiconductor
layer 26.
FIG. 35C is partly different from FIG. 35A. In FIG. 35C, the same
portions as those of FIG. 35A other than different portions are
denoted by the same reference numerals.
FIG. 35C illustrates an example which differs from FIG. 35A in
positional relation between source and drain regions 27a and 27b
and source and drain electrode layers 28a and 28b. The source and
drain regions 27a and 27b are provided below the source and drain
layers 28a and 28b. The source and drain layers 28a and 28b have an
effect of reducing plasma damage to the source and drain regions
27a and 27b.
In other words, as a blocking layer for reducing plasma damage to
the source and drain regions 27a and 27b, four layers (the source
and drain electrode layers 28a and 28b, the channel protective
layer 42, the resin layer 17, and the second gate electrode layer
19) are formed over the source and drain regions 27a and 27b;
therefore, plasma damage to the source and drain regions 27a and
27b is further reduced.
As for a thin film transistor 35 illustrated in FIG. 35C, an oxide
semiconductor film having low resistance is formed over and in
contact with the gate insulating layer 13 and a conductive film is
formed thereover. After that, the oxide semiconductor film having
low resistance is etched using the same resist mask as that used
for selectively etching the conductive film. Therefore, top
surfaces of the source and drain regions 27a and 27b which are
formed by etching the oxide semiconductor film having low
resistance have approximately the same shape as top surfaces of the
source and drain electrode layers 28a and 28b which are formed over
the source and drain regions 27a and 27b. The top surfaces and side
surfaces of the source and drain electrode layers 28a and 28b are
formed in contact with the oxide semiconductor layer 26.
FIG. 35D is partly different from FIG. 35C. In FIG. 35D, the same
portions as those of FIG. 35C other than different portions are
denoted by the same reference numerals.
FIG. 35D illustrates an example in which the second gate electrode
layer 19 and the second protective insulating layer 18 are formed
in an order different from FIG. 35C.
As illustrated in FIG. 35D, the second gate electrode layer 19 of a
thin film transistor 36 is formed over and in contact with the
resin layer 17 that is the first protective insulating film and
provided between the resin layer 17 and the second protective
insulating layer 18. In the case where the second gate electrode
layer 19 is provided between the resin layer 17 and the second
protective insulating layer 18, the second gate electrode layer 19
as well as the channel protective layer 42 and the resin layer 17
has an effect of reducing plasma damage to the oxide semiconductor
layer 26.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 13
FIG. 36A is an example of a cross-sectional view of a thin film
transistor in which an oxide semiconductor layer is sandwiched
between two gate electrode layers provided over and below the oxide
semiconductor layer. This embodiment describes an example of a thin
film transistor used for a pixel portion and a driver circuit which
are provided over a substrate having an insulating surface.
Note that this embodiment is the same as Embodiment 9 except that
an amorphous silicon film is provided in contact with an oxide
semiconductor layer 26. Therefore, detailed description is omitted
here and the same portions as those of FIG. 32A are denoted by the
same reference numerals. Steps are the same as those of Embodiment
9 up to formation of the oxide semiconductor layer 26.
Then, after forming the oxide semiconductor layer 26 according to
Embodiment 9, an amorphous semiconductor film or a compound thereof
obtained by a sputtering method, typically an amorphous silicon
film is formed without exposure to the air as a channel protective
layer 43 over and in contact with the oxide semiconductor layer 26.
Note that a compound of an amorphous silicon film refers to a
p-type amorphous silicon film formed by a sputtering method which
includes a p-type impurity element such as boron, or an n-type
amorphous silicon film formed by a sputtering method which includes
an n-type impurity element such as phosphorus.
In order to reduce damage to the oxide semiconductor layer 26 as
much as possible, the film is formed under a low power condition or
a condition where a substrate temperature is lower than 200.degree.
C. In this embodiment, the amorphous silicon film including boron
is formed with a substrate temperature set at room temperature and
electric power set at 1 kw.
In addition, before formation of the amorphous silicon film
including boron, the exposed region of the oxide semiconductor
layer may be subjected to oxygen radical treatment. By the oxygen
radical treatment, a surface and its vicinity of the oxide
semiconductor layer can be modified into an oxygen-excess region.
If the amorphous silicon film is formed on the oxygen-excess region
formed by the oxygen radical treatment, a thin film of SiO.sub.x
(X>0) is formed at an interface, which can reduce off
current.
Oxygen radicals may be produced in a plasma generation apparatus
with the use of a gas including oxygen, or in an ozone generation
apparatus. By exposing a thin film to the produced oxygen radicals
or oxygen, the surface of the film can be modified. The radical
treatment is not limited to one using oxygen radicals, and may be
performed using argon and oxygen radicals. The treatment using
argon and oxygen radicals is treatment in which an argon gas and an
oxygen gas are introduced to generate plasma, thereby modifying a
surface of a thin film.
The channel protective layer 43 can be used as an interlayer film
which blocks moisture, hydrogen ions, OH.sup.-, or the like. In
addition, the channel protective layer 43 formed of the amorphous
silicon film functions as a light-blocking layer for blocking
incidence of light to the oxide semiconductor layer.
Then, heat treatment at 200.degree. C. to 600.degree. C., typically
300.degree. C. to 500.degree. C., is preferably performed. In this
embodiment, heat treatment is performed in a furnace at 350.degree.
C. for 1 hour in a nitrogen atmosphere including oxygen.
Then, the resin layer 17 is formed to a thickness of in a range of
0.5 .mu.m to 3 .mu.M to cover the channel protective layer 43. As a
photosensitive or non photosensitive organic material for the resin
layer 17, polyimide, acrylic, polyamide, polyimideamide, resist,
benzocyclobutene, or a stack of any of these materials is used.
Note that steps after formation of the resin layer 17 are the same
as those of Embodiment 9 and therefore, are briefly described
here.
The second protective insulating layer 18 is formed to a thickness
of 50 nm to 400 nm by a PCVD method or a sputtering method under a
low power condition (or at a low substrate temperature of lower
than 200.degree. C., preferably a room temperature to 100.degree.
C.). Alternatively, the second protective insulating layer 18 may
be formed under a low power condition using a high-density plasma
apparatus.
After a conductive layer is formed, a photolithography step is
performed to form a resist mask over the conductive layer and
unnecessary portions are removed by etching so that wirings and
electrodes (wirings including a second gate electrode layer 19 and
the like) are formed.
Through the above process, a thin film transistor 37 illustrated in
FIG. 36A can be obtained.
FIG. 36B is partly different from FIG. 36A. In FIG. 36B, the same
portions as those of FIG. 36A other than different portions are
denoted by the same reference numerals.
FIG. 36B illustrates an example in which the second gate electrode
layer 19 and the second protective insulating layer 18 are formed
in an order different from FIG. 36A.
As illustrated in FIG. 36B, the second gate electrode layer 19 of a
thin film transistor 38 is formed over and in contact with the
resin layer 17 that is the first protective insulating film and
provided between the resin layer 17 and the second protective
insulating layer 18. In the case where the second gate electrode
layer 19 is provided between the resin layer 17 and the second
protective insulating layer 18, the second gate electrode layer 19
as well as the channel protective layer 43 and the resin layer 17
has an effect of reducing plasma damage to the oxide semiconductor
layer 26.
FIG. 36C is partly different from FIG. 36A. In FIG. 36C, the same
portions as those of FIG. 36A other than different portions are
denoted by the same reference numerals.
FIG. 36C illustrates an example which differs from FIG. 36A in
positional relation between source and drain regions and source and
drain electrode layers. The source and drain regions 27a and 27b
are provided below the source and drain layers 28a and 28b. The
source and drain layers 28a and 28b have an effect of reducing
plasma damage to the source and drain regions 27a and 27b.
In other words, as a blocking layer for reducing plasma damage to
the source and drain regions 27a and 27b, four layers (the source
and drain electrode layers 28a and 28b, the channel protective
layer 43, the resin layer 17, and the second gate electrode layer
19) are formed over the source and drain regions 27a and 27b;
therefore, plasma damage to the source and drain regions 27a and
27b is further reduced.
As for a thin film transistor 39 illustrated in FIG. 36C, an oxide
semiconductor film having low resistance is formed over and in
contact with the gate insulating layer 13 and a conductive film is
formed thereover. After that, the oxide semiconductor film having
low resistance is etched using the same resist mask as that used
for selectively etching the conductive film. Therefore, top
surfaces of the source and drain regions 27a and 27b which are
formed by etching the oxide semiconductor film having low
resistance have approximately the same shape as top surfaces of the
source and drain electrode layers 28a and 28b which are formed over
the source and drain regions 27a and 27b. The top surfaces and side
surfaces of the source and drain electrode layers 28a and 28b are
formed in contact with the oxide semiconductor layer 26.
FIG. 36D is partly different from FIG. 36C. In FIG. 36D, the same
portions as those of FIG. 36C other than different portions are
denoted by the same reference numerals.
FIG. 36D illustrates an example in which the second gate electrode
layer 19 and the second protective insulating layer 18 are formed
in an order different from FIG. 36C.
As illustrated in FIG. 36D, the second gate electrode layer 19 of a
thin film transistor 40 is formed over and in contact with the
resin layer 17 that is the first protective insulating film and
provided between the resin layer 17 and the second protective
insulating layer 18. In the case where the second gate electrode
layer 19 is provided between the resin layer 17 and the second
protective insulating layer 18, the second gate electrode layer 19
as well as the channel protective layer 43 and the resin layer 17
has an effect of reducing plasma damage to the oxide semiconductor
layer 26.
This embodiment can be implemented in an appropriate combination
any of with the structures described in the other embodiments.
Embodiment 14
An example of a display device which is one example of a
semiconductor device will be described below. In the display
device, at least part of a driver circuit and a thin film
transistor to be arranged in a pixel portion are formed over one
substrate.
The thin film transistor arranged in the pixel portion is formed
according to Embodiment 2, a channel formation region is formed
using an oxide semiconductor layer including SiO.sub.x, and source
and drain regions are formed using an oxide semiconductor to which
nitrogen is added. The thin film transistor is an n-channel TFT;
therefore, part of a driver circuit which can be formed using
n-channel TFTs is formed over the same substrate as the thin film
transistor in the pixel portion.
FIG. 17A illustrates an example of a block diagram of an active
matrix liquid crystal display device which is an example of a
semiconductor device. The display device illustrated in FIG. 17A
includes, over a substrate 5300, a pixel portion 5301 including a
plurality of pixels that are each provided with a display element;
a scan line driver circuit 5302 that selects a pixel; and a signal
line driver circuit 5303 that controls a video signal input to the
selected pixel.
In addition, the thin film transistor described in Embodiment 2 is
an n-channel TFT, and a signal line driver circuit including the
n-channel TFT is described with reference to FIG. 18.
The signal line driver circuit in FIG. 18 includes a driver IC
5601, switch groups 5602_1 to 5602_M, a first wiring 5611, a second
wiring 5612, a third wiring 5613, and wirings 5621_1 to 5621_M.
Each of the switch groups 5602_1 to 5602_M includes a first thin
film transistor 5603a, a second thin film transistor 5603b, and a
third thin film transistor 5603c.
The driver IC 5601 is connected to the first wiring 5611, the
second wiring 5612, the third wiring 5613, and the wirings 5621_1
to 5621_M. Each of the switch groups 5602_1 to 5602_M is connected
to the first wiring 5611, the second wiring 5612, and the third
wiring 5613, and the wirings 5621_1 to 5621_M are connected to the
switch groups 5602_1 to 5602_M, respectively. Each of the wirings
5621_1 to 5621_M is connected to three signal lines via the first
thin film transistor 5603a, the second thin film transistor 5603b,
and the third thin film transistor 5603c. For example, the wiring
5621_J of the J-th column (one of the wirings 5621_1 to 5621_M) is
connected to a signal line Sj-1, a signal line Sj, and a signal
line Sj+1 via the first thin film transistor 5603a, the second thin
film transistor 5603b, and the third thin film transistor 5603c
which are included in the switch group 5602_J.
A signal is input to each of the first wiring 5611, the second
wiring 5612, and the third wiring 5613.
Note that the driver IC 5601 is preferably formed over a single
crystalline substrate. Further, the switch groups 5602_1 to 5602_M
are preferably formed over the same substrate as the pixel portion
is. Therefore, the driver IC 5601 and the switch groups 5602_1 to
5602_M are preferably connected through an FPC or the like.
Next, operation of the signal line driver circuit shown in FIG. 18
is described with reference to a timing chart in FIG. 19. The
timing chart in FIG. 19 illustrates a case where the scan line Gi
of the i-th row is selected. A selection period of the scan line Gi
of the i-th row is divided into a first sub-selection period T1, a
second sub-selection period T2, and a third sub-selection period
T3. In addition, the signal line driver circuit in FIG. 18 operates
similarly to that in FIG. 19 even when a scan line of another row
is selected.
Note that the timing chart in FIG. 19 shows a case where the wiring
5621_J of the J-th column is connected to the signal line Sj-1, the
signal line Sj, and the signal line Sj+1 via the first thin film
transistor 5603a, the second thin film transistor 5603b, and the
third thin film transistor 5603c.
The timing chart in FIG. 19 shows timing at which the scan line Gi
of the i-th row is selected, timing 5703a of on/off of the first
thin film transistor 5603a, timing 5703b of on/off of the second
thin film transistor 5603b, timing 5703c of on/off of the third
thin film transistor 5603c, and a signal 5721_J input to the wiring
5621_J of the J-th column.
In the first sub-selection period T1, the second sub-selection
period T2, and the third sub-selection period T3, different video
signals are input to the wirings 5621_1 to 5621_M. For example, a
video signal input to the wiring 5621_J in the first sub-selection
period T1 is input to the signal line Sj-1, a video signal input to
the wiring 5621_J in the second sub-selection period T2 is input to
the signal line Sj, and a video signal input to the wiring 5621_J
in the third sub-selection period T3 is input to the signal line
Sj+1. In addition, the video signals input to the wiring 5621_J in
the first sub-selection period T1, the second sub-selection period
T2, and the third sub-selection period T3 are denoted by Data_j-1,
Data_j, and Data_j+1.
As illustrated in FIG. 19, in the first sub-selection period T1,
the first thin film transistor 5603a is turned on, and the second
thin film transistor 5603b and the third thin film transistor 5603c
are turned off. At this time, Data_j-1 input to the wiring 5621_J
is input to the signal line Sj-1 via the first thin film transistor
5603a. In the second sub-selection period T2, the second thin film
transistor 5603b is turned on, and the first thin film transistor
5603a and the third thin film transistor 5603c are turned off. At
this time, Data_j input to the wiring 5621_J is input to the signal
line Sj via the second thin film transistor 5603b. In the third
sub-selection period T3, the third thin film transistor 5603c is
turned on, and the first thin film transistor 5603a and the second
thin film transistor 5603b are turned off. At this time, Data_j+1
input to the wiring 5621_J is input to the signal line Sj+1 via the
third thin film transistor 5603c.
As described above, in the signal line driver circuit in FIG. 18,
by dividing one gate selection period into three, video signals can
be input to three signal lines from one wiring 5621 during one gate
selection period. Therefore, in the signal line driver circuit in
FIG. 18, the number of connections between the substrate provided
with the driver IC 5601 and the substrate provided with the pixel
portion can be approximately 1/3 of the number of signal lines. The
number of connections is reduced to approximately 1/3 of the number
of the signal lines, so that reliability, yield, etc., of the
signal line driver circuit in FIG. 18 can be improved.
Note that there are no particular limitations on the arrangement,
the number, a driving method, and the like of the thin film
transistors, as long as one gate selection period is divided into a
plurality of sub-selection periods and video signals are input to a
plurality of signal lines from one wiring in the respective
sub-selection periods as illustrated in FIG. 18.
For example, when video signals are input to three or more signal
lines from one wiring in three or more sub-selection periods, it is
only necessary to add a thin film transistor and a wiring for
controlling the thin film transistor. Note that when one gate
selection period is divided into four or more sub-selection
periods, one sub-selection period becomes shorter. Therefore, one
gate selection period is preferably divided into two or three
sub-selection periods.
As another example, one selection period may be divided into a
pre-charge period Tp, the first sub-selection period T1, the second
sub-selection period T2, and the third sub-selection period T3 as
illustrated in a timing chart in FIG. 20. The timing chart in FIG.
20 shows the timing at which the scan line Gi of the i-th row is
selected, timing 5803a at which the first thin film transistor
5603a is turned on/off, timing 5803b at which the second thin film
transistor 5603b is turned on/off, timing 5803c at which the third
thin film transistor 5603c is turned on/off, and a signal 5821_J
input to the wiring 5621_J of the J-th column. As illustrated in
FIG. 20, the first thin film transistor 5603a, the second thin film
transistor 5603b, and the third thin film transistor 5603c are
tuned on in the precharge period Tp. At this time, precharge
voltage Vp input to the wiring 5621_J is input to each of the
signal line Sj-1, the signal line Sj, and the signal line Sj+1 via
the first thin film transistor 5603a, the second thin film
transistor 5603b, and the third thin film transistor 5603c. In the
first sub-selection period T1, the first thin film transistor 5603a
is turned on, and the second thin film transistor 5603b and the
third thin film transistor 5603c are turned off. At this time,
Data_j-1 input to the wiring 5621_J is input to the signal line
Sj-1 via the first thin film transistor 5603a. In the second
sub-selection period T2, the second thin film transistor 5603b is
turned on, and the first thin film transistor 5603a and the third
thin film transistor 5603c are turned off. At this time, Data_j
input to the wiring 5621_J is input to the signal line Sj via the
second thin film transistor 5603b. In the third sub-selection
period T3, the third thin film transistor 5603c is turned on, and
the first thin film transistor 5603a and the second thin film
transistor 5603b are turned off. At this time, Data_j+1 input to
the wiring 5621_J is input to the signal line Sj+1 via the third
thin film transistor 5603c.
As described above, in the signal-line driver circuit of FIG. 18,
to which the timing chart of FIG. 20 is applied, a signal line can
be pre-charged by providing a pre-charge selection period before
sub-selection periods. Thus, a video signal can be written to a
pixel at a high speed. Note that portions in FIG. 20 which are
similar to those of FIG. 19 are denoted by common reference
numerals and detailed description of the portions which are the
same and portions which have similar functions is omitted.
Further, a structure of a scan line driver circuit is described.
The scan line driver circuit includes a shift register and a
buffer. Additionally, the scan line driver circuit may include a
level shifter in some cases. In the scan line driver circuit, when
the clock signal (CLK) and the start pulse signal (SP) are input to
the shift register, a selection signal is generated. The generated
selection signal is buffered and amplified by the buffer, and the
resulting signal is supplied to a corresponding scan line. Gate
electrodes of transistors in pixels of one line are connected to
the scan line. Since the transistors in the pixels of one line have
to be turned on all at once, a buffer which can supply a large
current is used.
One mode of a shift register which is used for part of a scan line
driver circuit is described with reference to FIG. 21 and FIG.
22.
FIG. 21 illustrates a circuit configuration of the shift register.
The shift register illustrated in FIG. 21 includes a plurality of
flip-flops: flip-flops 5701_1 to 5701.sub.--n. The shift register
is operated with input of a first clock signal, a second clock
signal, a start pulse signal, and a reset signal.
Connection relations of the shift register in FIG. 21 are
described. In the i-th stage flip-flop 5701.sub.--i (one of the
flip-flops 5701_1 to 5701.sub.--n) in the shift register of FIG.
21, a first wiring 5501 illustrated in FIG. 22 is connected to a
seventh wiring 5717.sub.--i-1; a second wiring 5502 illustrated in
FIG. 22 is connected to a seventh wiring 5717.sub.--i+1; a third
wiring 5503 illustrated in FIG. 22 is connected to a seventh wiring
5717.sub.--i; and a sixth wiring 5506 illustrated in FIG. 22 is
connected to a fifth wiring 5715.
Further, a fourth wiring 5504 illustrated in FIG. 22 is connected
to a second wiring 5712 in flip-flops of odd-numbered stages, and
is connected to a third wiring 5713 in flip-flops of even-numbered
stages. A fifth wiring 5505 illustrated in FIG. 22 is connected to
a fourth wiring 5714.
Note that the first wiring 5501 in FIG. 22, of the first stage
flip-flop 5701_1 is connected to a first wiring 5711. Moreover, the
second wiring 5502 in FIG. 22, of the n-th stage flip-flop
5701.sub.--n is connected to a sixth wiring 5716.
Note that the first wiring 5711, the second wiring 5712, the third
wiring 5713, and the sixth wiring 5716 may be referred to as a
first signal line, a second signal line, a third signal line, and a
fourth signal line, respectively. The fourth wiring 5714 and the
fifth wiring 5715 may be referred to as a first power supply line
and a second power supply line, respectively.
Next, FIG. 22 illustrates details of the flip-flop illustrated in
FIG. 21. A flip-flop illustrated in FIG. 22 includes a first thin
film transistor 5571, a second thin film transistor 5572, a third
thin film transistor 5573, a fourth thin film transistor 5574, a
fifth thin film transistor 5575, a sixth thin film transistor 5576,
a seventh thin film transistor 5577, and an eighth thin film
transistor 5578. Each of the first thin film transistor 5571, the
second thin film transistor 5572, the third thin film transistor
5573, the fourth thin film transistor 5574, the fifth thin film
transistor 5575, the sixth thin film transistor 5576, the seventh
thin film transistor 5577, and the eighth thin film transistor 5578
is an n-channel transistor and is turned on when the gate-source
voltage (V.sub.gs) exceeds the threshold voltage (V.sub.th).
In FIG. 22, a gate electrode of the third thin film transistor 5573
is electrically connected to the power supply line. Further, it can
be said that a circuit in which the third thin film transistor 5573
is connected to the fourth thin film transistor 5574 (a circuit
surrounded by the dotted line in FIG. 22) corresponds to a
configuration illustrated in FIG. 14A. Although the example in
which all the thin film transistors are enhancement type n-channel
transistors is described here, there is no limitation to this
example. For example, the driver circuit can be driven even with
the use of an n-channel depletion-mode transistor as the third thin
film transistor 5573.
Next, connections of the flip-flop shown in FIG. 21 are described
below.
A first electrode (one of a source electrode and a drain electrode)
of the first thin film transistor 5571 is connected to the fourth
wiring 5504. A second electrode (the other of the source electrode
and the drain electrode) of the first thin film transistor 5571 is
connected to the third wiring 5503.
A first electrode of the second thin film transistor 5572 is
connected to the sixth wiring 5506. A second electrode of the
second thin film transistor 5572 is connected to the third wiring
5503.
A first electrode of the third thin film transistor 5573 is
connected to the fifth wiring 5505, and a second electrode of the
third thin film transistor 5573 is connected to a gate electrode of
the second thin film transistor 5572. A gate electrode of the third
thin film transistor 5573 is connected to the fifth wiring
5505.
A first electrode of the fourth thin film transistor 5574 is
connected to the sixth wiring 5506. A second electrode of the
fourth thin film transistor 5574 is connected to a gate electrode
of the second thin film transistor 5572. A gate electrode of the
fourth thin film transistor 5574 is connected to a gate electrode
of the first thin film transistor 5571.
A first electrode of the fifth thin film transistor 5575 is
connected to the fifth wiring 5505. A second electrode of the fifth
thin film transistor 5575 is connected to the gate electrode of the
first thin film transistor 5571. A gate electrode of the fifth thin
film transistor 5575 is connected to the first wiring 5501.
A first electrode of the sixth thin film transistor 5576 is
connected to the sixth wiring 5506. A second electrode of the sixth
thin film transistor 5576 is connected to the gate electrode of the
first thin film transistor 5571. A gate electrode of the sixth thin
film transistor 5576 is connected to the gate electrode of the
second thin film transistor 5572.
A first electrode of the seventh thin film transistor 5577 is
connected to the sixth wiring 5506. A second electrode of the
seventh thin film transistor 5577 is connected to the gate
electrode of the first thin film transistor 5571. A gate electrode
of the seventh thin film transistor 5577 is connected to the second
wiring 5502. A first electrode of the eighth thin film transistor
5578 is connected to the sixth wiring 5506. A second electrode of
the eighth thin film transistor 5578 is connected to the gate
electrode of the second thin film transistor 5572. A gate electrode
of the eighth thin film transistor 5578 is connected to the first
wiring 5501.
Note that the points at which the gate electrode of the first thin
film transistor 5571, the gate electrode of the fourth thin film
transistor 5574, the second electrode of the fifth thin film
transistor 5575, the second electrode of the sixth thin film
transistor 5576, and the second electrode of the seventh thin film
transistor 5577 are connected are each referred to as a node 5543.
The points at which the gate electrode of the second thin film
transistor 5572, the second electrode of the third thin film
transistor 5573, the second electrode of the fourth thin film
transistor 5574, the gate electrode of the sixth thin film
transistor 5576, and the second electrode of the eighth thin film
transistor 5578 are connected are each referred to as a node
5544.
Note that the first wiring 5501, the second wiring 5502, the third
wiring 5503, and the fourth wiring 5504 may be referred to as a
first signal line, a second signal line, a third signal line, and a
fourth signal line, respectively. The fifth wiring 5505 and the
sixth wiring 5506 may be referred to as a first power supply line
and a second power supply line, respectively.
In addition, when the channel width of the transistor in the scan
line driver circuit is increased or a plurality of scan line driver
circuits are provided, for example, higher frame frequency can be
realized. When a plurality of scan line driver circuits are
provided, a scan line driver circuit for driving scan lines of
even-numbered rows is provided on one side and a scan line driver
circuit for driving scan lines of odd-numbered rows is provided on
the opposite side; thus, an increase in frame frequency can be
realized. Furthermore, the use of the plurality of scan line driver
circuits for output of signals to the same scan line is
advantageous in increasing the size of a display device.
Further, when an active matrix light-emitting display device which
is an example of a semiconductor device is manufactured, a
plurality of thin film transistors are arranged in at least one
pixel, and thus a plurality of scan line driver circuits are
preferably arranged. FIG. 17B is a block diagram illustrating an
example of an active matrix light-emitting display device.
The light-emitting display device illustrated in FIG. 17B includes,
over a substrate 5400, a pixel portion 5401 having a plurality of
pixels each provided with a display element, a first scan line
driver circuit 5402 and a second scan line driver circuit 5404 that
select a pixel, and a signal line driver circuit 5403 that controls
input of a video signal to the selected pixel.
When the video signal input to a pixel of the light-emitting
display device illustrated in FIG. 17B is a digital signal, a pixel
is in a light-emitting state or in a non-light-emitting state by
switching of on and off of a transistor. Thus, grayscale can be
displayed using an area grayscale method or a time grayscale
method. An area grayscale method refers to a driving method in
which one pixel is divided into a plurality of subpixels and the
respective subpixels are driven independently based on video
signals so that grayscale is displayed. Further, a time grayscale
method refers to a driving method in which a period during which a
pixel emits light is controlled so that grayscale is displayed.
Since the response time of a light-emitting element is higher than
that of a liquid crystal element or the like, the light-emitting
element is more suitable for a time grayscale method than the
liquid crystal element. Specifically, in the case of displaying
with a time grayscale method, one frame period is divided into a
plurality of subframe periods. Then, in accordance with video
signals, the light-emitting element in the pixel is brought into a
light-emitting state or a non-light-emitting state in each subframe
period. By dividing one frame period into a plurality of subframe
periods, the total length of time, in which a pixel actually emits
light in one frame period, can be controlled by video signals so
that grayscale can be displayed.
In the example of the light-emitting display device illustrated in
FIG. 17B, in a case where two switching TFTs are arranged in one
pixel, the first scan line driver circuit 5402 generates a signal
which is input to a first scan line serving as a gate wiring of one
of the switching TFTs, and the second scan line driver circuit 5404
generates a signal which is input to a second scan line serving as
a gate wiring of the other of the switching TFTs; however, one scan
line driver circuit may generate both the signal which is input to
the first scan line and the signal which is input to the second
scan line. In addition, for example, there is a possibility that a
plurality of scan lines used for controlling the operation of the
switching element are provided in each pixel, depending on the
number of the switching TFTs included in one pixel. In this case,
one scan line driver circuit may generate all signals that are
input to the plurality of scan lines, or a plurality of scan line
driver circuits may generate signals that are input to the
plurality of scan lines.
Also in the light-emitting display device, a part of a driver
circuit that can include n-channel TFTs among driver circuits can
be formed over the same substrate as the thin film transistors of
the pixel portion.
Moreover, the above-described driver circuit can be used for an
electronic paper that drives electronic ink using an element
electrically connected to a switching element, without being
limited to applications to a liquid crystal display device or a
light-emitting display device. The electronic paper is also
referred to as an electrophoretic display device (an
electrophoretic display) and is advantageous in that it has the
same level of readability as plain paper, it has lower power
consumption than other display devices, and it can be made thin and
lightweight.
Electrophoretic displays can have various modes. Electrophoretic
displays contain a plurality of microcapsules dispersed in a
solvent or a solute, each microcapsule including first particles
which are positively charged and second particles which are
negatively charged. By applying an electric field to the
microcapsules, the particles in the microcapsules move in opposite
directions to each other and only the color of the particles
gathering on one side is displayed. Note that the first particles
and the second particles each include pigment and do not move
without an electric field. Moreover, the first particles and the
second particles have different colors (which may be
colorless).
Thus, an electrophoretic display is a display that utilizes a
so-called dielectrophoretic effect by which a substance having a
high dielectric constant moves to a high-electric field region. An
electrophoretic display device does not need to use a polarizer or
a counter substrate, which is required in a liquid crystal display
device, and both the thickness and weight of the electrophoretic
display device can be reduced to a half of those of a liquid
crystal display device.
A solution in which the above microcapsules are dispersed in a
solvent is referred to as electronic ink. This electronic ink can
be printed on a surface of glass, plastic, cloth, paper, or the
like. Furthermore, by using a color filter or particles that have a
pigment, color display can also be achieved.
In addition, if a plurality of the above microcapsules is arranged
as appropriate over an active matrix substrate so as to be
interposed between two electrodes, an active matrix display device
can be completed, and display can be performed by application of an
electric field to the microcapsules. For example, the active matrix
substrate including the thin film transistor (in which an oxide
semiconductor layer including SiO.sub.x is used for a channel
formation region and an oxide semiconductor to which nitrogen is
added is used for source and drain regions) described in Embodiment
2 can be used.
Note that the first particles and the second particles in the
microcapsules may each be formed of a single material selected from
a conductive material, an insulating material, a semiconductor
material, a magnetic material, a liquid crystal material, a
ferroelectric material, an electroluminescent material, an
electro-chromic material, and a magnetophoretic material, or formed
of a composite material of any of these.
Through the above process, a highly reliable display device as a
semiconductor device can be manufactured.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 15
This embodiment describes an example of a light-emitting display
device as a semiconductor device. As a display element included in
a display device, a light-emitting element utilizing
electroluminescence is described here. Light-emitting elements
utilizing electroluminescence are classified according to whether a
light emitting material is an organic compound or an inorganic
compound. The former is referred to as an organic EL element and
the latter is referred to as an inorganic EL element.
In an organic EL element, by application of voltage to a
light-emitting element, electrons and holes are separately injected
from a pair of electrodes into a layer including a light-emitting
organic compound, and current flows. The carriers (electrons and
holes) are recombined, and thus, the light-emitting organic
compound is excited. The light-emitting organic compound returns to
a ground state from the excited state, thereby emitting light.
Owing to such a mechanism, this light-emitting element is referred
to as a current-excitation light-emitting element.
The inorganic EL elements are classified according to their element
structures into a dispersion-type inorganic EL element and a
thin-film inorganic EL element. A dispersion-type inorganic EL
element has a light-emitting layer where particles of a
light-emitting material are dispersed in a binder, and its light
emission mechanism is donor-acceptor recombination type light
emission that utilizes a donor level and an acceptor level. A
thin-film inorganic EL element has a structure where a
light-emitting layer is sandwiched between dielectric layers, which
are further sandwiched between electrodes, and its light emission
mechanism is localized type light emission that utilizes
inner-shell electron transition of metal ions. Note that an example
of an organic EL element as a light-emitting element is described
here.
FIG. 23 illustrates an example of a pixel structure to which
digital time grayscale driving can be applied, as an example of a
semiconductor device.
A structure and operation of a pixel to which digital time
grayscale driving can be applied are described. Here, one pixel
includes two n-channel transistors in each of which an oxide
semiconductor layer including SiO.sub.x (typically, a Zn--O-based
non-single-crystal film) is used for a channel formation region and
a Zn--O-based oxide semiconductor to which nitrogen is added is
used for source and drain regions.
A pixel 6400 includes a switching transistor 6401, a driver
transistor 6402, a light-emitting element 6404, and a capacitor
6403. A gate of the switching transistor 6401 is connected to a
scan line 6406, a first electrode (one of a source electrode and a
drain electrode) of the switching transistor 6401 is connected to a
signal line 6405, and a second electrode (the other of the source
electrode and the drain electrode) of the switching transistor 6401
is connected to a gate of the driver transistor 6402. The gate of
the driver transistor 6402 is connected to a power supply line 6407
via the capacitor 6403, a first electrode of the driver transistor
6402 is connected to the power supply line 6407, and a second
electrode of the driver transistor 6402 is connected to a first
electrode (pixel electrode) of the light-emitting element 6404. A
second electrode of the light-emitting element 6404 corresponds to
a common electrode 6408. The common electrode 6408 is electrically
connected to a common potential line provided over the same
substrate, and the connection portion may be used as a common
connection portion.
The second electrode (common electrode 6408) of the light-emitting
element 6404 is set to a low power supply potential. Note that the
low power supply potential is a potential satisfying the low power
supply potential<a high power supply potential with reference to
the high power supply potential that is set to the power supply
line 6407. As the low power supply potential, GND, 0 V, or the like
may be employed, for example. A potential difference between the
high power supply potential and the low power supply potential is
applied to the light-emitting element 6404 and current is supplied
to the light-emitting element 6404, so that the light-emitting
element 6404 emits light. Here, in order to make the light-emitting
element 6404 emit light, each potential is set so that the
potential difference between the high power supply potential and
the low power supply potential is a forward threshold voltage or
higher of the light-emitting element 6404.
Note that gate capacitor of the driver transistor 6402 may be used
as a substitute for the capacitor 6403, so that the capacitor 6403
can be omitted. The gate capacitor of the driver transistor 6402
may be formed between the channel region and the gate
electrode.
In the case of a voltage-input voltage driving method, a video
signal is input to the gate of the driver transistor 6402 so that
the driver transistor 6402 is in either of two states of being
sufficiently turned on or turned off. That is, the driver
transistor 6402 operates in a linear region. Since the driver
transistor 6402 operates in the linear region, a voltage higher
than the voltage of the power supply line 6407 is applied to the
gate of the driver transistor 6402. Note that a voltage higher than
or equal to (voltage of the power supply line+Vth of the driver
transistor 6402) is applied to the signal line 6405.
Further, in the case of using analog grayscale driving instead of
the digital time ratio grayscale driving, the pixel structure the
same as that of FIG. 23 can be employed by inputting signals in a
different way.
In the case of performing analog grayscale driving, a voltage
higher than or equal to (forward voltage of the light-emitting
element 6404+Vth of the driver transistor 6402) is applied to the
gate of the driver transistor 6402. The forward voltage of the
light-emitting element 6404 indicates a voltage at which a desired
luminance is obtained, and includes at least forward threshold
voltage. The video signal by which the driver transistor 6402
operates in a saturation region is input, so that current can be
supplied to the light-emitting element 6404. In order for the
driver transistor 6402 to operate in the saturation region, the
potential of the power supply line 6407 is set higher than the gate
potential of the driver transistor 6402. When an analog video
signal is used, it is possible to feed current to the
light-emitting element 6404 in accordance with the video signal and
perform analog grayscale driving.
Note that a pixel structure of the present invention is not limited
to that shown in FIG. 23. For example, a switch, a resistor, a
capacitor, a transistor, a logic circuit, or the like may be added
to the pixel shown in FIG. 23.
Next, structures of the light-emitting element will be described
with reference to FIGS. 24A to 24C. Here, a cross-sectional
structure of a pixel will be described by taking an n-channel
driving TFT as an example. Driving TFTs 7001, 7011, and 7021 used
for semiconductor devices illustrated in FIGS. 24A to 24C can be
formed in a manner similar to formation of the thin film transistor
170 described in Embodiment 2 and are thin film transistors in each
of which an oxide semiconductor layer including SiO.sub.x for a
channel formation region and an oxide semiconductor to which
nitrogen is added is used for source and drain regions.
In order to extract light emitted from the light-emitting element,
at least one of an anode and a cathode is required to transmit
light. A thin film transistor and a light-emitting element are
formed over a substrate. A light-emitting element can have a top
emission structure, in which light emission is extracted through
the surface opposite to the substrate; a bottom emission structure,
in which light emission is extracted through the surface on the
substrate side; or a dual emission structure, in which light
emission is extracted through the surface opposite to the substrate
and the surface on the substrate side. The pixel structure can be
applied to a light-emitting element having any of these emission
structures.
A light-emitting element having a top emission structure will be
described with reference to FIG. 24A.
FIG. 24A is a cross-sectional view of a pixel in the case where the
driving TFT 7001 is an n-channel TFT and light is emitted from a
light-emitting element 7002 to an anode 7005 side. In the TFT 7001,
a Zn--O-based oxide semiconductor to which silicon oxide is added
is used for a semiconductor layer and a Zn--O-based oxide
semiconductor to which nitrogen is added is used for source and
drain regions. In FIG. 24A, a cathode 7003 of the light-emitting
element 7002 is electrically connected to the driving TFT 7001, and
a light-emitting layer 7004 and the anode 7005 are stacked in this
order over the cathode 7003. The cathode 7003 can be formed using a
variety of conductive materials as long as they have a low work
function and reflect light. For example, Ca, Al, MgAg, AlLi, or the
like is preferably used. The light-emitting layer 7004 may be
formed using a single layer or a plurality of layers stacked. When
the light-emitting layer 7004 is formed using a plurality of
layers, the light-emitting layer 7004 is formed by stacking an
electron-injecting layer, an electron-transporting layer, a
light-emitting layer, a hole-transporting layer, and a
hole-injecting layer in this order over the cathode 7003. It is not
necessary to form all of these layers. The anode 7005 is formed
using a light-transmitting conductive film such as a film of indium
oxide including tungsten oxide, indium zinc oxide including
tungsten oxide, indium oxide including titanium oxide, indium tin
oxide including titanium oxide, indium tin oxide (hereinafter
referred to as ITO), indium zinc oxide, or indium tin oxide to
which silicon oxide is added.
The light-emitting element 7002 corresponds to a region where the
light-emitting layer 7004 is sandwiched between the cathode 7003
and the anode 7005. In the case of the pixel illustrated in FIG.
24A, light is emitted from the light-emitting element 7002 to the
anode 7005 side as indicated by an arrow.
Next, a light-emitting element having a bottom emission structure
will be described with reference to FIG. 24B. FIG. 21B is a
cross-sectional view of a pixel in the case where the driving TFT
7011 is of an n-type and light is emitted from a light-emitting
element 7012 to a cathode 7013 side. In the TFT 7011, an
In--Zn--O-based oxide semiconductor to which silicon oxide is added
is used for a semiconductor layer and an In--Zn--O-based oxide
semiconductor to which nitrogen is added is used for source and
drain regions. In FIG. 24B, the cathode 7013 of the light-emitting
element 7012 and the TFT 7011 that is a driving TFT are
electrically connected to each other, and a light-emitting layer
7014 and an anode 7015 are stacked in this order over the cathode
7013. A light-blocking film 7016 for reflecting or blocking light
may be formed to cover the anode 7015 when the anode 7015 has a
light-transmitting property. For the cathode 7013, a variety of
materials can be used as in the case of FIG. 24A as long as they
are conductive materials having a low work function. The cathode
7013 is formed to have a thickness that can transmit light
(preferably, approximately 5 nm to 30 nm). For example, an aluminum
film with a thickness of 20 nm can be used as the cathode 7013.
Similar to the case of FIG. 24A, the light-emitting layer 7014 may
be formed using either a single layer or a plurality of layers
stacked. The anode 7015 is not required to transmit light, but can
be formed using a light-transmitting conductive material as in the
case of FIG. 24A. As the light-blocking film 7016, a metal or the
like that reflects light can be used for example; however, it is
not limited to a metal film. For example, a resin or the like to
which black pigments are added can also be used.
The light-emitting element 7012 corresponds to a region where the
light-emitting layer 7014 is sandwiched between the cathode 7013
and the anode 7015. In the case of the pixel illustrated in FIG.
24B, light is emitted from the light-emitting element 7012 to the
cathode 7013 side as indicated by an arrow.
Next, a light-emitting element having a dual emission structure
will be described with reference to FIG. 24C. In FIG. 24C, a
cathode 7023 of a light-emitting element 7022 is formed over a
light-transmitting conductive film 7027 which is electrically
connected to the driving TFT 7021, and a light-emitting layer 7024
and an anode 7025 are stacked in this order over the cathode 7023.
In the TFT 7021, a Zn--O-based oxide semiconductor to which silicon
oxide is added is used for a semiconductor layer and a Zn--O-based
oxide semiconductor to which nitrogen is added is used for source
and drain regions. As in the case of FIG. 24A, the cathode 7023 can
be formed using a variety of conductive materials as long as they
have a low work function. The cathode 7023 is formed to have a
thickness that can transmit light. For example, a film of Al having
a thickness of 20 nm can be used as the cathode 7023. As in FIG.
24A, the light-emitting layer 7024 may be formed using either a
single layer or a plurality of layers stacked. The anode 7025 can
be formed using a light-transmitting conductive material as in the
case of FIG. 24A.
The light-emitting element 7022 corresponds to a region where the
cathode 7023, the light-emitting layer 7024, and the anode 7025
overlap with one another. In the case of the pixel illustrated in
FIG. 24C, light is emitted from the light-emitting element 7022 to
both the anode 7025 side and the cathode 7023 side as indicated by
arrows.
Note that, although the organic EL elements are described here as
the light-emitting elements, an inorganic EL element can also be
provided as a light-emitting element.
In this embodiment, the example is described in which a thin film
transistor (a driving TFT) which controls the driving of a
light-emitting element is electrically connected to the
light-emitting element; however, a structure may be employed in
which a TFT for current control is connected between the driving
TFT and the light-emitting element.
Next, the appearance and cross section of a light-emitting display
panel (also referred to as a light-emitting panel) which
corresponds to one mode of a semiconductor device are described
with reference to FIGS. 25A and 25B. FIG. 25A is a top view of a
panel in which thin film transistors and a light-emitting element,
which are formed over a first substrate, are sealed between the
first substrate and a second substrate with a sealant. FIG. 25B
corresponds to a cross-sectional view taken along line H-I of FIG.
25A.
A sealant 4505 is provided so as to surround a pixel portion 4502,
signal line driver circuits 4503a and 4503b, and scan line driver
circuits 4504a and 4504b which are provided over a first substrate
4501. In addition, a second substrate 4506 is provided over the
pixel portion 4502, the signal line driver circuits 4503a and
4503b, and the scan line driver circuits 4504a and 4504b.
Accordingly, the pixel portion 4502, the signal line driver
circuits 4503a and 4503b, and the scan line driver circuits 4504a
and 4504b are sealed together with a filler 4507, by the first
substrate 4501, the sealant 4505, and the second substrate 4506. It
is preferable that a panel be packaged (sealed) with a protective
film (such as a laminate film or an ultraviolet curable resin film)
or a cover material with high air-tightness and little
degasification so that the panel is not exposed to the outside air,
in this manner.
The pixel portion 4502, the signal line driver circuits 4503a and
4503b, and the scan line driver circuits 4504a and 4504b formed
over the first substrate 4501 each include a plurality of thin film
transistors, and the thin film transistor 4510 included in the
pixel portion 4502 and the thin film transistor 4509 included in
the signal line driver circuit 4503a are illustrated as an example
in FIG. 25B.
In each of the thin film transistors 4509 and 4510, a Zn--O-based
oxide semiconductor to which silicon oxide is added is used, and a
Zn--O-based oxide semiconductor to which nitrogen is added is used
for source and drain regions. In this embodiment, the thin film
transistors 4509 and 4510 are n-channel thin film transistors.
Moreover, reference numeral 4511 denotes a light-emitting element.
A first electrode layer 4517 which is a pixel electrode included in
the light-emitting element 4511 is electrically connected to a
source electrode layer or a drain electrode layer of the thin film
transistor 4510. Note that a structure of the light-emitting
element 4511 is a stacked-layer structure of the first electrode
layer 4517, the electroluminescent layer 4512, and the second
electrode layer 4513, but there is no particular limitation on the
structure. The structure of the light-emitting element 4511 can be
changed as appropriate depending on the direction in which light is
extracted from the light-emitting element 4511, or the like.
A partition 4520 is formed using an organic resin film, an
inorganic insulating film, or organic polysiloxane. It is
particularly preferable that the partition 4520 be formed using a
photosensitive material and an opening be formed over the first
electrode layer 4517 so that a sidewall of the opening is formed as
an inclined surface with continuous curvature.
The electroluminescent layer 4512 may be formed with a single layer
or a plurality of layers stacked.
A protective film may be formed over the second electrode layer
4513 and the partition 4520 in order to prevent entry of oxygen,
hydrogen, moisture, carbon dioxide, or the like into the
light-emitting element 4511. As the protective film, a silicon
nitride film, a silicon nitride oxide film, a DLC film, or the like
can be formed.
In addition, a variety of signals and potentials are supplied to
the signal line driver circuits 4503a and 4503b, the scan line
driver circuits 4504a and 4504b, or the pixel portion 4502 from
FPCs 4518a and 4518b.
In this embodiment, a connection terminal electrode 4515 is formed
from the same conductive film as the first electrode layer 4517
included in the light-emitting element 4511, and a terminal
electrode 4516 is formed from the same conductive film as the
source and drain electrode layers included in the thin film
transistors 4509 and 4510.
The connection terminal electrode 4515 is electrically connected to
a terminal included in the FPC 4518a via an anisotropic conductive
film 4519.
As the second substrate located in the direction in which light is
extracted from the light-emitting element 4511 needs to have a
light-transmitting property. In that case, a light-transmitting
material such as a glass plate, a plastic plate, a polyester film,
or an acrylic film is used for the second substrate.
As the filler 4507, an ultraviolet curable resin or a thermosetting
resin can be used, in addition to an inert gas such as nitrogen or
argon. For example, PVC (polyvinyl chloride), acrylic, polyimide,
an epoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA
(ethylene vinyl acetate) can be used. In this embodiment, nitrogen
is used for the filler.
In addition, if needed, an optical film, such as a polarizing
plate, a circularly polarizing plate (including an elliptically
polarizing plate), a retardation plate (a quarter-wave plate or a
half-wave plate), or a color filter, may be provided as appropriate
on a light-emitting surface of the light-emitting element. Further,
the polarizing plate or the circularly polarizing plate may be
provided with an anti-reflection film. For example, anti-glare
treatment by which reflected light can be diffused by projections
and depressions on the surface so as to reduce the glare can be
performed.
The signal line driver circuits 4503a and 4503b and the scanning
line driver circuits 4504a and 4504b may be mounted as driver
circuits formed using a single crystal semiconductor film or a
polycrystalline semiconductor film over a substrate separately
prepared. In addition, only the signal line driver circuits or part
thereof, or the scan line driver circuits or part thereof may be
separately formed and mounted. This embodiment is not limited to
the structure illustrated in FIGS. 25A and 25B.
Through the above process, a highly reliable light-emitting display
device (display panel) as a semiconductor device can be
manufactured.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 16
Thin film transistors in each of which an oxide semiconductor layer
including silicon oxide (SiO.sub.x) is used for a channel formation
region and an oxide semiconductor to which nitrogen is added is
used for source and drain regions are formed, and a liquid crystal
display device having a display function, in which the thin film
transistors are included in a driver circuit and a pixel portion,
can be manufactured. Further, part or whole of a driver circuit can
be formed over the same substrate as a pixel portion, using a thin
film transistor, whereby a system-on-panel can be obtained.
The liquid crystal display device includes a liquid crystal element
(also referred to as a liquid crystal display element) as a display
element.
Further, a liquid crystal display device includes a panel in which
a liquid crystal display element is sealed, and a module in which
an IC or the like including a controller is mounted to the panel.
An embodiment of the present invention also relates to an element
substrate, which corresponds to one mode before the display element
is completed in a manufacturing process of the liquid crystal
display device, and the element substrate is provided with means
for supplying current to the display element in each of a plurality
of pixels. Specifically, the element substrate may be in a state in
which only a pixel electrode of the display element is provided, a
state after formation of a conductive film to be a pixel electrode
and before etching of the conductive film to form the pixel
electrode, or any other states.
Note that a liquid crystal display device in this specification
means an image display device, a display device, or a light source
(including a lighting device). Further, the liquid crystal display
device includes any of the following modules in its category: a
module to which a connector such as an FPC (flexible printed
circuit), TAB (tape automated bonding) tape, or a TCP (tape carrier
package) is attached; a module having TAB tape or a TCP which is
provided with a printed wiring board at the end thereof; and a
module having an IC (integrated circuit) directly mounted on a
substrate provided with a display element by a COG (chip on glass)
method.
The appearance and a cross section of a liquid crystal display
panel, which is one embodiment of a liquid crystal display device,
will be described with reference to FIGS. 26A-1, 26A-2, and 26B.
FIGS. 26A-1 and 26A-2 are top views of panels in which a liquid
crystal element 4013 is sealed with a sealant 4005 between a first
substrate 4001 and a second substrate 4006. FIG. 26B is a
cross-sectional view taken along M-N of FIGS. 26A-1 and 26A-2.
The sealant 4005 is provided so as to surround a pixel portion 4002
and a scan line driver circuit 4004 which are provided over the
first substrate 4001. The second substrate 4006 is provided over
the pixel portion 4002 and the scan line driver circuit 4004.
Therefore, the pixel portion 4002 and the scan line driver circuit
4004 are sealed together with a liquid crystal layer 4008, by the
first substrate 4001, the sealant 4005, and the second substrate
4006. A blue-phase liquid crystal material is used for the liquid
crystal layer 4008 in this embodiment without particular
limitation. A liquid crystal material exhibiting a blue phase has a
short response time of 1 millisecond or less from the state of
applying no voltage to the state of applying voltage, whereby
short-time response is possible. The liquid crystal material
exhibiting a blue phase includes a liquid crystal and a chiral
agent. The chiral agent is employed to align the liquid crystal in
a helical structure and to make the liquid crystal exhibit a blue
phase. For example, a liquid crystal material into which a chiral
agent is mixed at 5 wt % or more may be used for the liquid crystal
layer. As a liquid crystal, a thermotropic liquid crystal, a low
molecular liquid crystal, a high molecular liquid crystal, a
ferroelectric liquid crystal, an anti-ferroelectric liquid crystal,
or the like is used.
In FIG. 26A-1, a signal line driver circuit 4003 that is formed
using a single crystal semiconductor film or a polycrystalline
semiconductor film over a substrate separately prepared is mounted
in a region that is different from the region surrounded by the
sealant 4005 over the first substrate 4001. In contrast, FIG. 26A-2
illustrates an example in which part of a signal line driver
circuit 4003 is formed over the first substrate 4001. A signal line
driver circuit 4003b is formed over the first substrate 4001 and a
signal line driver circuit 4003a which is formed using a single
crystal semiconductor film or a polycrystalline semiconductor film
is mounted on the substrate separately prepared.
Note that the connection method of a driver circuit which is
separately formed is not particularly limited, and a COG method, a
wire bonding method, a TAB method, or the like can be used. FIG.
26A-1 illustrates an example of mounting the signal line driver
circuit 4003 by a COG method, and FIG. 26A-2 illustrates an example
of mounting the signal line driver circuit 4003 by a TAB
method.
Each of the pixel portion 4002 and the scan line driver circuit
4004 which are provided over the first substrate 4001 includes a
plurality of thin film transistors. FIG. 26B illustrates a thin
film transistor 4010 included in the pixel portion 4002 and a thin
film transistor 4011 included in the scan line driver circuit 4004.
Over the thin film transistors 4010 and 4011, insulating layers
4020 and 4021 are provided. The thin film transistors 4010 and 4011
can be each a thin film transistor in which an oxide semiconductor
layer including silicon oxide (SiO.sub.x) is used for a channel
formation region and an oxide semiconductor to which nitrogen is
added is used for source and drain regions. In this embodiment, the
thin film transistors 4010 and 4011 are n-channel thin film
transistors.
A pixel electrode layer 4030 and a common electrode layer 4031 are
provided over the first substrate 4001, and the pixel electrode
layer 4030 is electrically connected to the thin film transistor
4010. The liquid crystal element 4013 includes the pixel electrode
layer 4030, the common electrode layer 4031, and the liquid crystal
layer 4008. In this embodiment, a method is used in which grayscale
is controlled by generating an electric field which is
substantially parallel to a substrate (i.e., in a lateral
direction) to move liquid crystal molecules in a plane parallel to
the substrate. In such a method, an electrode structure used in an
in plane switching (IPS) mode or a fringe field switching (FFS)
mode can be used. Note that a polarizing plate 4032 and a
polarizing plate 4033 are provided on the outer sides of the first
substrate 4001 and the second substrate 4006, respectively.
As the first substrate 4001 and the second substrate 4006, glass,
plastic, or the like having a light-transmitting property can be
used. As plastic, a fiberglass-reinforced plastics (FRP) plate, a
polyvinyl fluoride (PVF) film, a polyester film, or an acrylic
resin film can be used. In addition, a sheet with a structure in
which an aluminum foil is sandwiched between PVF films or polyester
films can be used.
A columnar spacer denoted by reference numeral 4035 is obtained by
selective etching of an insulating film and is provided in order to
control the thickness (a cell gap) of the liquid crystal layer
4008. Alternatively, a spherical spacer may also be used.
FIGS. 26A-1 and 26A-2, and 26B illustrate examples of liquid
crystal display devices in which a polarizing plate is provided on
the outer side (the view side) of a substrate; however, the
polarizing plate may be provided on the inner side of the
substrate. The position of the polarizing plate may be determined
as appropriate depending on the material of the polarizing plate
and conditions of the manufacturing process. Furthermore, a
light-blocking layer serving as a black matrix may be provided.
The insulating layer 4021 that is an interlayer film is a
light-transmitting resin layer. Part of the interlayer film 4021
may be a light-blocking layer. It is preferable that the
light-blocking layer is provided to cover the thin film transistors
4010 and 4011. In FIGS. 26A-1 and 26A-2, and 26B, a light-blocking
layer 4034 is provided on the second substrate 4006 side so as to
cover the thin film transistors 4010 and 4011. By the
light-blocking layer 4012 and the light-blocking layer 4034,
further improvement in contrast and in stabilization of the thin
film transistors can be achieved.
When the light-blocking layer 4034 is provided, the intensity of
incident light on the semiconductor layers of the thin film
transistors can be attenuated. Accordingly, electric
characteristics of the thin film transistors can be stabilized and
prevented from being varied due to photosensitivity of the oxide
semiconductor.
The thin film transistors may be covered with the insulating layer
4020 which serves as a protective film of the thin film
transistors; however, there is no particular limitation to such a
structure.
Note that the protective film is provided to prevent entry of
contaminant impurities such as organic substance, metal, or
moisture existing in air and is preferably a dense film. The
protective film may be formed with a single layer or a stacked
layer of a silicon oxide film, a silicon nitride film, a silicon
oxynitride film, a silicon nitride oxide film, an aluminum oxide
film, an aluminum nitride film, aluminum oxynitride film, and/or an
aluminum nitride oxide film by a sputtering method.
Further, in the case of further forming a light-transmitting
insulating layer as a planarizing insulating film, the
light-transmitting insulating layer can be formed using an organic
material having heat resistance, such as polyimide, acrylic,
benzocyclobutene, polyamide, or epoxy. Other than such organic
materials, it is also possible to use a low-dielectric constant
material (a low-k material), a siloxane-based resin, PSG
(phosphosilicate glass), BPSG (borophosphosilicate glass), or the
like. The insulating layer may be formed by stacking a plurality of
insulating films formed of these materials.
There is no particular limitation on the formation method of the
insulating layer having a stacked structure, and the following
method can be employed in accordance with the material: sputtering,
an SOG method, spin coating, dip coating, spray coating, droplet
discharging (for example, ink jetting, screen printing, or offset
printing), doctor knife, roll coating, curtain coating, knife
coating, or the like. In the case where the insulating layer is
formed using a material solution, the semiconductor layers may be
annealed (at 200.degree. C. to 400.degree. C.) at the same time of
a baking step. The baking step of the insulating layer also serves
as the annealing step of the semiconductor layer, whereby a liquid
crystal display device can be manufactured efficiently.
The pixel electrode layer 4030 and the common electrode layer 4031
can be formed using a light-transmitting conductive material such
as indium oxide including tungsten oxide, indium zinc oxide
including tungsten oxide, indium oxide including titanium oxide,
indium tin oxide including titanium oxide, indium tin oxide
(hereinafter referred to as ITO), indium zinc oxide, or indium tin
oxide to which silicon oxide is added.
A conductive composition including a conductive high molecule (also
referred to as a conductive polymer) can be used for the pixel
electrode layer 4030 and the common electrode layer 4031.
Further, a variety of signals and a potential are supplied to the
signal line driver circuit 4003 which is formed separately, the
scan line driver circuit 4004, and the pixel portion 4002 from an
FPC 4018.
Since a thin film transistor is easily broken due to static
electricity or the like, a protective circuit for protecting the
driver circuit is preferably provided over the same substrate for a
gate line or a source line. The protective circuit is preferably
formed with a non-linear element including an oxide
semiconductor.
In FIGS. 26A-1 and 26A-2 and FIG. 26B, a connecting terminal
electrode 4015 is formed using the same conductive film as the
pixel electrode layer 4030, and a terminal electrode 4016 is formed
using the same conductive film as source and drain electrode layers
of the thin film transistors 4010 and 4011.
The connection terminal electrode 4015 is electrically connected to
a terminal included in the FPC 4018 via an anisotropic conductive
film 4019.
Further, FIGS. 26A-1 and 26A-2 and FIG. 26B illustrate an example
in which the signal line driver circuit 4003 is formed separately
and mounted on the first substrate 4001; however, this embodiment
is not limited to this structure. The scan line driver circuit may
be separately formed and then mounted, or only part of the signal
line driver circuit or part of the scan line driver circuit may be
separately formed and then mounted.
FIG. 27 illustrates an example of a cross-sectional structure of a
liquid crystal display device in which an element layer 2603
including TFTs and the like and a liquid crystal layer 2604 are
provided between an element substrate 2600 and a counter substrate
2601 which are bonded with a sealant 2602.
In the case where color display is performed, light-emitting diodes
which emit light of plural colors are arranged in a backlight
portion. In the case of an RGB mode, a red light-emitting diode
2910R, a green light-emitting diode 2910G, and a blue
light-emitting diode 2910B are disposed in each of the regions into
which a display area of the liquid crystal display device is
divided.
A polarizing plate 2606 is provided on the outer side of the
counter substrate 2601, and a polarizing plate 2607 and an optical
sheet 2613 are provided on the outer side of the element substrate
2600. A light source is formed using the red light-emitting diode
2910R, the green light-emitting diode 2910G, the blue
light-emitting diode 2910B, and a reflective plate 2611. An LED
control circuit 2912 provided for a circuit substrate 2612 is
connected to a wiring circuit portion 2608 of the element substrate
2600 through a flexible wiring board 2609 and further includes an
external circuit such as a control circuit or a power source
circuit.
This embodiment describes a field-sequential liquid crystal display
device in which the LEDs are individually made to emit light by
this LED control circuit 2912 without particular limitation. It is
also possible to use a cold cathode fluorescent lamp or a white LED
as a light source of the backlight and to provide a color
filter.
Further, this embodiment employs an electrode structure used in an
in plane switching (IPS) mode without particular limitation. A
twisted nematic (TN) mode, a multi-domain vertical alignment (MVA)
mode, a patterned vertical alignment (PVA) mode, an axially
symmetric aligned micro-cell (ASM) mode, an optical compensated
birefringence (OCB) mode, a ferroelectric liquid crystal (FLC)
mode, an antiferroelectric liquid crystal (AFLC) mode, or the like
can be used.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 17
This embodiment describes an example of electronic paper as a
semiconductor device.
FIG. 28A is a cross-sectional view of active-matrix electronic
paper. A thin film transistor 581 provided in a display portion of
a semiconductor device can be manufactured in a manner similar to
the thin film transistor described in Embodiment 2, in which an
oxide semiconductor layer including silicon oxide (SiO.sub.x) is
used for a channel formation region and an oxide semiconductor to
which nitrogen is added is used for source and drain regions.
The electronic paper in FIG. 28A is an example of a display device
using a twisting ball display method. The twisting ball display
system refers to a method in which spherical particles each colored
in black and white are arranged between a first electrode layer and
a second electrode layer which are electrode layers used for a
display element, and a potential difference is generated between
the first electrode layer and the second electrode layer to control
orientation of the spherical particles, so that display is
performed.
The thin film transistor 581 sealed between the substrate 580 and
the substrate 596 is a thin film transistor with a bottom-gate
structure, and a source or drain electrode layer thereof is in
contact with a first electrode layer 587 through an opening formed
in insulating layer 585, whereby the thin film transistor 581 is
electrically connected to the first electrode layer 587. Between
the first electrode layer 587 and a second electrode layer 588,
spherical particles 589 each having a black region 590a, a white
region 590b, and a cavity 594 around the regions which is filled
with liquid are provided. A space around the spherical particles
589 is filled with a filler 595 such as a resin (see FIG. 28A). In
this embodiment, the first electrode layer 587 corresponds to a
pixel electrode, and the second electrode layer 588 corresponds to
a common electrode. The second electrode layer 588 is electrically
connected to a common potential line provided over the same
substrate as the thin film transistor 581. With the use of a common
connection portion, the second electrode layer 588 can be
electrically connected to the common potential line through
conductive particles provided between a pair of substrates.
Further, instead of the twisting ball, an electrophoretic element
can also be used. A microcapsule having a diameter of approximately
10 .mu.m to 200 .mu.m in which transparent liquid, positively
charged white microparticles, and negatively charged black
microparticles are encapsulated, is used. In the microcapsule which
is provided between the first electrode layer and the second
electrode layer, when an electric field is applied by the first
electrode layer and the second electrode layer, the white
microparticles and the black microparticles move to opposite sides,
so that white or black can be displayed. A display element using
this principle is an electrophoretic display element, and is
generally called an electronic paper. The electrophoretic display
element has higher reflectance than a liquid crystal display
element, and thus, an auxiliary light is unnecessary, power
consumption is low, and a display portion can be recognized in a
dim place. In addition, even when power is not supplied to the
display portion, an image which has been displayed once can be
maintained. Accordingly, a displayed image can be stored even when
a semiconductor device having a display function (which may be
referred to simply as a display device or a semiconductor device
provided with a display device) is distanced from an electric wave
source.
With use of the thin film transistor formed by the steps described
in Embodiment 2, in which an oxide semiconductor layer including
silicon oxide is used for a channel formation region and an oxide
semiconductor to which nitrogen is added is used for source and
drain regions, electronic paper can be manufactured with reduced
manufacturing cost, as a semiconductor device. Electronic paper can
be used for electronic appliances of a variety of fields as long as
they can display data. For example, electronic paper can be applied
to an e-book reader (electronic book), a poster, an advertisement
in a vehicle such as a train, or displays of various cards such as
a credit card. An example of the electronic apparatus is
illustrated in FIG. 28B.
FIG. 28B illustrates an example of an e-book 2700. For example, the
e-book reader 2700 includes two housings, a housing 2701 and a
housing 2703. The housing 2701 and the housing 2703 are combined
with a hinge 2711 so that the e-book reader 2700 can be opened and
closed with the hinge 2711 as an axis. With such a structure, the
e-book reader 2700 can operate like a paper book.
A display portion 2705 and a display portion 2707 are incorporated
in the housing 2701 and the housing 2703, respectively. The display
portion 2705 and the display portion 2707 may display one image or
different images. In the case where the display portion 2705 and
the display portion 2707 display different images, for example, a
display portion on the right side (the display portion 2705 in FIG.
28B) can display text whereas a display portion on the left side
(the display portion 2707 in FIG. 28B) can display graphics.
In the example illustrated in FIG. 28B, the housing 2701 is
provided with an operation portion and the like. For example, the
housing 2701 is provided with a power switch 2721, an operation key
2723, a speaker 2725, and the like. With the operation key 2723,
pages can be turned. Note that a keyboard, a pointing device, and
the like may be provided on the same surface as the display portion
of the housing. Furthermore, an external connection terminal (an
earphone terminal, a USB terminal, a terminal that can be connected
to various cables such as an AC adapter and a USB cable, or the
like), a recording medium insertion portion, and the like may be
provided on the back surface or the side surface of the housing.
Moreover, the e-book reader 2700 may have a function of an
electronic dictionary.
The e-book reader 2700 may have a structure capable of wirelessly
transmitting and receiving data. Through wireless communication,
desired book data or the like can be purchased and downloaded from
an electronic book server.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
Embodiment 18
A semiconductor device including a thin film transistor in which an
oxide semiconductor layer including silicon oxide (SiO.sub.x) is
used for a channel formation region and an oxide semiconductor to
which nitrogen is added is used for source and drain regions can be
applied to a variety of electronic appliances (including amusement
machines). Examples of electronic devices are a television set
(also referred to as a television or a television receiver), a
monitor of a computer or the like, a camera such as a digital
camera or a digital video camera, a digital photo frame, a mobile
phone handset (also referred to as a mobile phone or a mobile phone
device), a portable game console, a portable information terminal,
an audio reproducing device, a large-sized game machine such as a
pachinko machine, and the like.
FIG. 29A illustrates an example of a television set 9600. In the
television set 9600, a display portion 9603 is incorporated in a
housing 9601. The display portion 9603 can display images. Here,
the back of the housing 9601 is supported so that the television
set 9600 is fixed to a wall.
The television set 9600 can be operated with an operation switch of
the housing 9601 or a separate remote controller 9610. Channels and
volume can be controlled with an operation key 9609 of the remote
controller 9610 so that an image displayed on the display portion
9603 can be controlled. Furthermore, the remote controller 9610 may
be provided with a display portion 9607 for displaying data output
from the remote controller 9610.
Note that the television set 9600 is provided with a receiver, a
modem, and the like. With the use of the receiver, general
television broadcasting can be received. Moreover, when the display
device is connected to a communication network with or without
wires via the modem, one-way (from a sender to a receiver) or
two-way (between a sender and a receiver or between receivers)
information communication can be performed.
FIG. 29B is a portable amusement machine and includes two housings,
a housing 9881 and a housing 9891, which are connected with a joint
portion 9893 so that the portable amusement machine can be opened
or folded. A display portion 9882 and a display portion 9883 are
incorporated in the housing 9881 and the housing 9891,
respectively. In addition, the portable amusement machine
illustrated in FIG. 29B includes a speaker portion 9884, a
recording medium insert portion 9886, an LED lamp 9890, an input
means (an operation key 9885, a connection terminal 9887, a sensor
9888 (a sensor having a function of measuring force, displacement,
position, speed, acceleration, angular velocity, rotational
frequency, distance, light, liquid, magnetism, temperature,
chemical substance, sound, time, hardness, electric field, current,
voltage, electric power, radiation, flow rate, humidity, gradient,
oscillation, odor, or infrared rays), or a microphone 9889), and
the like. Needless to say, the structure of the portable amusement
machine is not limited to the above and other structures provided
with at least a semiconductor device may be employed. The portable
amusement machine may include other accessory equipment, as
appropriate. The portable amusement machine illustrated in FIG. 29B
has a function of reading a program or data stored in a recording
medium to display it on the display portion, and a function of
sharing information with another portable amusement machine via
wireless communication. The portable amusement machine in FIG. 29B
can have various functions such as, but not limited to, a function
to the above.
FIG. 30A illustrates an example of a mobile phone 1000. The mobile
phone 1000 includes a display portion 1002 incorporated in a
housing 1001, an operation button 1003, an external connection port
1004, a speaker 1005, a microphone 1006 and the like.
Data can be input to the mobile phone 1000 illustrated in FIG. 30A
by touching the display portion 1002 with a finger or the like.
Furthermore, operations such as making calls and composing mails
can be performed by touching the display portion 1002 with a finger
or the like.
There are mainly three screen modes of the display portion 1002.
The first mode is a display mode mainly for displaying images. The
second mode is an input mode mainly for inputting data such as
text. The third mode is a display-and-input mode in which two modes
of the display mode and the input mode are combined.
For example, in a case of making a call or composing a mail, a text
input mode mainly for inputting text is selected for the display
portion 1002 so that text displayed on a screen can be input. In
that case, it is preferable to display a keyboard or number buttons
on almost all area of the screen of the display portion 1002.
When a detection device including a sensor for detecting
inclination, such as a gyroscope or an acceleration sensor, is
provided inside the mobile phone 1000, display in the screen of the
display portion 1002 can be automatically switched by determining
the installation direction of the mobile phone 1000 (whether the
mobile phone 1000 is placed horizontally or vertically for a
landscape mode or a portrait mode).
The screen modes are switched by touching the display portion 1002
or operating the operation button 1003 of the housing 1001.
Alternatively, the screen modes may be switched in accordance with
the kind of the image displayed on the display portion 1002. For
example, when a signal of an image displayed on the display portion
is a signal of moving image data, the screen mode is switched to
the display mode. When the signal is a signal of text data, the
screen mode is switched to the input mode.
Further, in the input mode, when input by touching the display
portion 1002 is not performed for a certain period while a signal
detected by the optical sensor in the display portion 1002 is
detected, the screen mode may be controlled so as to be switched
from the input mode to the display mode.
The display portion 1002 may function as an image sensor. For
example, an image of a palm print, a fingerprint, or the like is
taken when the display portion 1002 is touched with a palm or a
finger, whereby personal identification can be performed. Further,
by provision of a backlight or a sensing light source which emits a
near-infrared light in the display portion, an image of a finger
vein, a palm vein, or the like can be taken.
FIG. 30B also illustrates an example of a mobile phone. The mobile
phone illustrated in FIG. 30B is provided with a display device
9410 having a display portion 9412 and operation buttons 9413 in a
housing 9411 and a communication device 9400 having an operation
buttons 9402, an external input terminal 9403, a microphone 9404, a
speaker 9405, and a light-emitting portion 9406 which emits light
when receiving a call in a housing 9401. The display device 9410
having a display function can be detached from or attached to the
communication device 9400 having a telephone function in two
directions indicated by arrows. Accordingly, the display device
9410 and the communication device 9400 can be attached to each
other along their short sides or long sides. When only the display
function is needed, the display device 9410 can be detached from
the communication device 9400 and used alone. Images or input
information can be transmitted or received by wireless or wired
communication between the communication device 9400 and the display
device 9410, each of which has a rechargeable battery.
This embodiment can be implemented in an appropriate combination
with any of the structures described in the other embodiments.
This application is based on Japanese Patent Application serial No.
2009-077386 filed with Japan Patent Office on Mar. 26, 2009, the
entire contents of which are hereby incorporated by reference.
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